Is the F8 key stuck down as if it's being pressed? If so, I'd just go buy another keyboard. I think you can pick one up for under $20.00.

On the other hand, you might just try a hard shut down and restart the computer. Unless the F8 key is stuck as though it's being pressed it should start up in regular mode.

To do a hard shut down just push the power button on the tower and hold it for around 5-6 seconds and it will shut down.

Sorry if I'm getting too basic. It's hard to explain without knowing your skill level.

HTH

If the F8 is just stuck then pull out the key and hit the rubber underneath with a pen instead. Otherwise as suggested get another keyboard