While I was showering (I had been for a while) my shower turned off and now it won't turn back on. What is wrong with it?

Hi Q...

You need to start at the source here... ;)

Shut off the water supply and then disassemble the shower valve... sounds like something got loose and clogged things. Diassemble the valve, check for loose parts and then cover the valve with a towel as you have someone turn the water supply back on for a couple seconds (or less) to confirm hot and cold water flow to the valve. This also acts to purge the valve of anything that might have clogged the pipes.

If that fails to reveal the issue then you may find the problem back at a shutoff in the water line(s).

Start there... let us know how you make out, OK?

Mark