Why is my calculator flashing validating OS

It keeps turning itself off and back on? The easiest thing to try is holding the reset button on the back of the calculator for at least two seconds.

If that doesn't work then the battery is probably nearly dead. Leave it plugged in overnight and it should start working again.

Unfortunately newer TI-84 Plus CE calculators have trouble charging dead batteries so leaving it plugged in may not work. In this case you can try charging the battery in a friend's calculator, or buy a new battery. If the calculator is under a year old then you can contact TI Cares and they may send you a new battery for free.