I know this question is quite popular but in my case I can't use any of the given advice.
I've accidentally deleted /tmp folder, my PC froze so I had to hard reboot it. Now it won't recognize the drive as bootable. What can I do to fix it?
I have a thumb drive with Ubuntu that I could use but I can't get to my drive with terminal.
I have noticed that too. Just recreate /tmp with sudo mkdir and it should boot again.
Of course you need another bootable device, for example, Ubuntu live USB. Then you should be able to find the damaged tree with lsblk or any similar tool.
/tmp directory permissions are also very important. See this: unix.stackexchange.com/questions/71622/…