I hadn't run any updates for over a month, so decided to run them. Once they were installed and I was told I was 'up to date', I restarted my laptop. Only it would not boot. After repeated attempts, I tried hitting F keys on boot-up, and saw a screen with this information: Failed to start Light Display Manager (Bad TLP) A start job is running for hold until boot process finishes up.
But the boot process never finishes; it gets hung up and Linux never loads.
Any advice? Thanks!

Please provide more information about your system.
Please boot into recovery mode (or something tot hat effect) and issue
show us that output.
Use code tags for that output.

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Thanks Ondoho, but I have no idea how to boot in recovery mode or use code tags. I am a true newbie. The computer is a Dell laptop and I installed Linux on it immediately after I bought it in around February. I do not use the computer that much, so I tend to allow the recommended updates to pile up. This time, when I initially tried to install the updates, several of the updates came back as no longer available. When I then refreshed/updated the updater program, it came back with a bunch of installed updates and said my computer was up-to-date. That was the last time I have been able to use my computer. If anyone can instruct me on how to provide my system information, I will gladly do so.
Thanks again.

Boot the system up, when it's done press Ctrl-Alt-F2 - does that bring you to a login prompt?

If yes, log in with your usual credentials and enter the following commands:
and give us the link that you get.

If no: https://www.startpage.com/do/dsearch...+into+recovery

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ondoho,
Thanks. Because my computer does not boot, I tried the Ctrl-Alt-F2 command a few seconds after turning on thhe computer. I did not get a login prompt. This is what I got:
Dell Inspiron 3576
Settings
General
System Information
Battery Information
Boot Sequence
Advanced Boot Options
UEFI Boot Path Security
Date/Time
System Configuration
Video
Security
Secure Boot
Performance
Power Management
POST Behavior
Virtualization Support
Wireless
Maintenance
System Logs
Support Assist System Resolution

Restore Settings Apply Exit

Please let me know if I performed the Ctrl-Alt-F2 procedure incorrectly and I will try it again.
Thanks.

I followed the link to instructions on how to Boot to Recovery Mode. I arrived at the following options:
resume
clean
dpkg
fsck
grub
network
root
system-summary

Which option should I select?
Thanks

After running the suggested pastebinit commands, here is the link I was given: https://pastebin.com/FbGuaGXN

So far so good?

OK.
your display manager, lightdm, errored out but it doesn't tell us why (you actually provided that info in post #1, I'm sorry it slipped by me).
Using the same method, please provide
you can also try to get to a text prompt with Ctrl-Alt-F2 again, but do it at the very end of the boot process (when lightdm errors).

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The response I got was: Unknown operation lightdm.
My guess is that one of the updates that I had originally attempted had something to do with lightdm, and that update probably never properly loaded.
Next step?
Thanks.

I cannot successfully do a Ctrl-Alt-F2 because upon boot, the screen will remain blank for hours, and I can only see what is on the screen when I power the machine off. Then it gives me a one-second glimpse at the screen. That is the only way I was able to see the "Failed to Start Light Display Mananger" and "Bad TLP" messages. And, like I posted earlier, I ultimately end up at the Dell Inspiron Settings screen.

Here's a site that speaks of the recovery mode that you got above. That site suggests signing in as root, and then mucking about, which frankly is hardly safe. I would first try "clean", and see what happens. Then, I'd try "fsck" (file system consistency check). Then "resume". Then, if it's not working still, "system-summary" to see if it gives you any clues (it likely won't). Finally, if that had failed, I'd choose "root" and attempt to enter in. If it lets you in to access the system (likely only by command line), then you can run the commands that ondoho suggested to get more feedback.

Don't guess. It was my mistake, should've been

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I googled bad TLP. Apparently it's caused by a hardware error on the pci bus in your computer.

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Hazel probably hit it on the nose. Hardware breaks.

I usually use a

after

in case something was missed during upgrade. I be looking for zero outputs.

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ondoho-
I tried the systemctl -n999 status lightdm code and this is what I got:

lightdm.service - Light Display Manager
Loaded: loaded (/lib/systemd/system/lightdm.service; indirect; vendor preset:
Active: inactive (dead)
Docs: man:lightdm(1)
log file:

At this point I was unable to use the lspci -k code, but when I returned to my root prompt and entered it, I was given a screen filled with information about kernel modules and kernel drivers

So, at this point do we have any useful information to diagnose or fix the problem? What about rokytnji's idea about apt-get? Might that be useful now?

Although hazel may be correct, it is odd that my hardware would break when the laptop has hardly been used in its meager six months of existence. And why would it break immediately after doing an overdue update? hmm

Mark alfred suggested I try other options besides root, such as "clean" and "fsck". When I select those options, I am told: "Continuing will remount your /filesystem in read/write mode and mount any filesystem defined in /etc/fstab. Do you wish to continue?
Because I don't understand what any of that means, I have thus far chosen not to proceed. But I'd happily try those options if proceeding is safe.

Thank you all! I sincerely hope our efforts here will bear some fruit shortly, as I have been stuck using a 1999 computer with no 'h' key and running firefox 35. A tad cumbersome.