1. Check the connection of the keyboard cable in the back of the computer and make sure it is tightly plugged in.  If not, plug it in.
2. Once you are sure that the keyboard is properly plugged into the back of the computer, power off the computer and then restart it.

The letters appearing on the screen do not match the ones being typed.

This type of problem can happen on programmable key boards.   These types of keyboards should have a Macro key and Suspend Macro key on the upper left corner of the keyboard.  Hold down the CTRL key, the Alt key, and the Suspend Macro key all at the same time.  This will clear any macros that have been programmed into the key board.