Have you looked for a run away daemon? Use the command.
ps -ef | grep mysql
You should see two lines, one with your grep command and the other with mysqld. Note the PID (Process ID). You will kill that daemon. (Exciting isn't it!). For the example, let's say that you find mysqld running at PID = 1234.
kill -1 1234
This is the mildest way to kill a process. See if the process has ended by running a ps command
ps -p 1234
If you get a blank response, then the daemon is dead.
If it doesn't work in a minute or two then escalate your kill:
kill -2 1234
Repeat the ps -p command. in a minute or two.
If you need to, try something sterner:
kill -15 1234
Now your pouring on the juice! Repeat you ps command.
If all else fails, and this is serious, then try
ps -9 1234. This command is a last ditch attack because it could leave files corrupted and lose data.
Of course, you could just reboot, if you don't feel like hunting for daemons.