Always first check to see if your caps lock is on. Another cause of password not working is keyboard related. 

Crumbs get stuck below the keys preventing the key from working. Unplug or turn off/remove batteries and hold the keyboard upside down rubbing your hand across the keys to dislodge anything.

If you're using a wireless keyboard, missing letters when you type and wrong password entries are often a sign of weak batteries (assuming the keyboard is clean).