Your screw is stripped, and your screwdriver is no longer sufficient. Before busting out the heavy hitters, try the following:

Use different screwdrivers. First try slightly smaller or larger head sizes, then a flat head screwdriver, to see if you can get it to catch on some part of the stripped screw.

If the bit doesn't catch the screw instantly, do not continue. Otherwise, you run the risk of further stripping the screw.

If either of these techniques are fruitful, congrats! Your screw is free.