Will the calculator turn on at all? If not, check the batteries first. There is a little coin battery that stores memory for the device behind where the regular batteries sit. This does not often fail, but could cause the thing to not power up. Note: removing the coin battery will clear any information stored on your calculator.
If the device is stuck in an infinite loop, or is unresponsive with the busy icon, try pressing and holding the "2nd", left, right and "on" buttons in that order and holding them all at the same time. This is the command to cancel whatever process the device is working and clear the internal memory (At least on the TI-89. I'm not 100% sure the command works on an 84).