When your server loses power unexpectedly, or the kernel panics or otherwise terminates unexpectedly, your mysql tables are probably going to be damaged.
You might not find out about it for a while. It is a good idea to check and repair all your databases before the problems get worse.
mysqlcheck -u root -p --auto-repair --check --optimize --all-databases
That command will fix your databases. I was seeing a lot of locks and queries failing that should not have failed, and this command fixed it for me.