You have not told us what OS version you are using. I'll assume 10.2.8 for this reply, instructions for 10.0, 10.1 or 10.3 should be virtually identical. OS 9 instructions are slightly different.
Open System Preferences -> Universal Access -> Mouse tab.
See if "Mouse Keys" has been turned on, that allows the numeric keypad to move the mouse pointer around. Next, try the Keyboard tab, see if Sticky Keys is on. This could simulate an input freeze while your system waits for the next modifier key to be pressed.
Also try System Preferences -> Keyboard & Mouse -> Keyboard Access.
See if "Full Keyboard Access" has been turned on, this can cause some keys to get assigned new functions.