My air condition unit keep running since last night. I turn the unit off at the thermostat, but I had to switch it off from the main box. I have a central unit.

If it is the inside blower running it sounds like the blower relay is stuck closed. Tap on it with a screwdriver handle and see it that will free it. Many times it will but it will need to be replaced. If it is the outside unit running, the contacts in the contactor are probably welded shut. Disconnect the power to inspect them. Asx a temp fix, it you can get them freed up, tyou can take a small file or finger nail file to them for a temp fix until you can get a replacement