LM 20.3 has been working fantastic then I got a warning about running out of space at 15.8GB. I turned off the computer planning to deal with it next session. The computer boots up as normal and when I enter my password, the screen blinks for a few seconds and then reappears. I tried on purpose to enter a bad password and it recognized incorrect password. A few search's said to try Alt+Ctrl+f1 at login to get to a different way to log in. So far no luck. This is the 1st time ever in Linux that I've not been able to sign in. So, I know my HDD is less tan half full, I usually keep 3 timeshift records and check them once in a while. First ever message that I was running out of space on something, essentially the exact same system forever. What to try? Thanks

I have a couple of thoughts.

I'd probably boot to a Live CD/USB of something, mount the relevant partition on the HDD, and check the log files to see it they gave any hint of what's going on. I'd start with /var/log/boot.log and /var/log/syslog.

While there, you might also check the disk usage to verify that you have sufficient free space.

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OK thanks. I still have the original USB from install. I'll try that and report back.

Depending on partition layout, sometimes, / (root filesystem) is on its own partition and separate from /home -- when this is the case, /var -- where the logs @frankbell is referring to are stored, can fill up this root partition with large logs -- or if many applications are installed, the apps fill up /. In either case, if it is so full that logs can no longer be written, or the processes in /var/run written, then login problems can occur. In addition to checking those logs, while running from the install media, you might want to check how full the paritions are with "df -h," and if / is full, you may want to resize it.

Ok I got logged in and got the warning that the whole file system only has 4GB left, I clicked explore from the warning and it brought up the disk usage analyzer and so far nothing big. I checked timeshift and it was fine but I deleated one of the 3 I saved to buy extra space but something is for the 1st time storing 30+GB that should be free. Still waiting for disk analyzer to finish but this being new, what is safe to simply delete and speed up the process? I thought Linux was suppose to handle all maintenance automatically and there was zero cleaning one had to do?

Well to sum up, I got the running out of disk space warning and then was unable to log in. Due to great advice here I booted to the original USB and was able to get access and look for the cause. Nothing in disk usage analyzer gave me a clue. I only have a 80GB HDD but still I was nowhere near capacity. So while I don't know exactly what fixed it, I did several things. 10 I ran sudo apt-get autoclean and sudo apt-get remove 2) sudo journalctl --vacuum-time=5d to clean the logs and 3) finally removed numerous small downloads and an old kernel I was no longer using. Still, none of these individually were more than 1GB so shouldn't have made much difference but it seems to have solved the problem and the computer is working great. Added up I free'd up about 7GB but when all was done I got 25GB back. Never did figure out where it was hiding. Of note is that its a very older computer but runs fast on Linux with several windows open. A question out of this experience is I though Linux did all of this for me, should I be doing some of this myself every few months? Thanks to all who helped.

Linux looks after itself. Logs get rotated and then removed so they don't take fill up the drive. When and how frequently varies according to which distribution. Occasionally a problem could fill up logs faster than the automated rotation and deletion, and it might need some help.

I learn by trial and error, and, trying to learn a new application, but configuring it incorrectly has filled my logs with errors taking up space way more quickly than it would had everything been correctly configured: more than once have I had to intervene and manually remove these logs. I found this out the same way you did: couldn't log in, because /var was full. I have /var on its own partition--also /tmp--because linux fills these spaces up without me, so I don't want them filling the entire drive. Booting from other media and manually removing large logs sorted it out each time.

But linux doesn't look after the user so much, because it doesn't know what the user wants to keep or throw away. Downloaded content and user installed content is at the users discretion to keep clean.

Thanks for the update. Glad you got it fixed!