The fact that your system froze is not indicative of anything specific. Could be many causes. This is a rare happening in Linux, but it does happen. Often times it is the "window manager" that freezes. Your entire system is not frozen, but it feels that way if you don't know what to do.
The overall plan is to:
(1) Get out of your locked state
(2) Reboot from a thumbdrive or a CD - anything but your normal hard drive (which we assume is possibly corrupted at this point)
(3) Run a "file systems check" on your hard disk
(4) Attempt to reboot from your hard disk again to see if everything is fixed
To address (1) above:
You can try opening up a new "console window". Your system should be configured to have several of these. You access them by holding down "control" and "shift" and a function key ... F2, F3, etc. You can try hitting ctrl-shift-F2 when things appear frozen and see if that brings up a new console window. A console window is just a login prompt. You can use your login/password to get a command prompt. But if you don't know the command line, then this hasn't done you any good. You can go back to your (frozen) GUI window with ctrl-shift-F7. It may not be F7 on your system, but it will be some function key. Try them all. I am running LinuxMint, same as you, and my GUI is on F7, so yours probably is too.
It's too involved to tell you what you can do from the command line in this single post if you don't already have some knowledge of that. Remember that when you bring up a new console window, you have to login first, before you are given a command prompt (which will probably end in a $ sign). Once you get that command prompt, you can blindly try this (without knowing what it's doing):
It will ask you for a password, enter yours, even though you just entered it a few seconds ago when you logged in.
You will now get another command line prompt, but this time it will probably end in a # sign, not a $ sign. Type the following:
Code:
sync;sync;sync;sleep 10;shutdown now
Sit back and do nothing more
for a minute or so and you should hear your system power down after that. Hopefully. If everything went as planned.
Now, this is hitting a locked up screen with a big hammer, but it's a safe hammer, and one that can be easily communicated. I would do things differently, but it's harder to explain what you would do (it's more interactive).
You got out of your locked state in a different way than I described above. You killed the power to your computer. Doing that could have corrupted your hard disk. Linux is very good at recovering from a "crash", which is what happens when you yank it's power cord. But it is not perfect and it can't always recover gracefully. I can count on one finger how many times I have seen a crash corrupt Linux, so it is very rare. But it does happen. One thing you can do to help protect yourself when you decide to do a human-initiated crash, is let the computer sit there, untouched, for five minutes before you yank it's power. That means don't touch the keyboard, don't move the mouse, just let it be as idle as possible. This gives the disk(s) time to sync if the system is still sane enough to run that operation as it would normally do on a routine basis.
To address (2) above:
You have already managed to solve this one. You can boot from a thumbdrive. There are lots of potential problems here. If you are booting UEFI, it may be set up for "Secure Boot". If you are booting legacy (BIOS), then it may be set to try booting from the hard drive before it tries to boot from the thumbdrive. Which defeats the purpose if your hard drive is temporarily dead. But, you said you got it booted from a thumbdrive, so you're all good. All the potential problems didn't affect your case. So you're in good shape.
I'm going to break this post in half and hit "Post reply" now. And then I'll post the second half of my troubleshooting tome in a followup post (which will take me a few minutes to type...