Squeeeeek-No-More-Floor-Repair-Kit stuck screw
The joist finder screw is designed so that it will go through the subfloor and spin freely if there is no joist.  Once you find a spot where it does not spin freely, you have found a joist.  If there is no joist, you have to pull up on the drill to get the screw out.  I assume the screw isn't spinning freely.

If the screw is totally stuck and will not move, you have most likely ran into a nail embedded in the joist.  So, the good news is you found a joist.  To remove, grab the screw with a pair of vice lock pliers as close to the floor as possible.  If you just grab the head, it it likely to break as the shaft flexes and twists.  Tapping it with a hammer could help loosen it, but don't hit it hard enough to bend it.

Either the screw will break, or you will still not be able to turn it.  If all else fails, you will have to cut it off with a Dremel tool or other similar cut off tool.  If it comes to this, it will be hard to cut without leaving a mark on the carpet.  Protect the immediate area with masking tape (like a 1 square foot area).  The remaining shaft can be pounded flat with a hammer.