Get a wrench, some masking tape, and some teflon tape (plumber's tape).
Wrap the masking tape 1 layer thick around the shower head nut and wrench the head off the pipe. The tape is to prevent scratching of the soft chrome.
Take the shower head over to your sink, and run water through it, backwards. You want to flush whatever silt and sand you might have in the screens. If this doesn't work, you may need to disassemble the head to get the rocks out. A paper clip and an old toothbrush can help.
On the other hand, a new shower head costs anywhere from $3 to $300...
When you go to reinstall the head, clean the threads on the pipe that sticks out of the wall, and wrap 3-4 wraps of teflon tape as flat as possible onto the threads. It should look like you painted the threads white when you are done. Standing in the shower, you should wrap the pipe in a CLOCKWISE fashion, so that when you screw the head on, it tightens the tape, instead of unwrapping it. Tighten it hand tight, then use your wrench to get 1/2 - 1 additional turns, and it shouldn't leak.
Keep in mind that most shower heads limit flow to somewhere between 1.5 and 2.5 gpm (gallons per minute) so the flow isn't going to be huge in any case.
The only reason you might have a broken pipe is if your water pressure is low in all or most of your faucets. Check your crawlspace for water, if you have one. I think you would probably know if you actually had a busted pipe.