This happens as operators are drilling through sand or gravel. When the operator stops circulation to add the next section of drill pipe, the cuttings settle around the bit. If this occurs, operators should allow extra time (as much as 10 minutes) to flush the cuttings and loosen the stuck bit. If this does not resolve the problem, try thickening the mud with bentonite to assist in removing the cuttings. For more help read here.
I had exactly the same problem with my drill and was about to give up on it. However, I noticed that it was a bit damp after being stored in the garage, so suspected that some moisture was affecting the electronics. Here's what I did to get it working again: I undid all the screws in the casing and separated the two halves. At the bottom by the battery connector is a small circuit board. I removed this (still attached to the wires) and used a hair drier to blow it with hot air all over for a few minutes. Make sure you blow all around the little chips - this is where I reckon the moisture was getting and upsetting the circuitry that controls the speed. more details