I stepped into the shower this morning and all was right with the world. I turned on the water and activated the shower head. Two seconds later, the water abruptly stopped. I deactivated the shower head and the water still came from the main faucet but not the head. Do you think the line broke?

We are talking about a tub/shower aren't we? If the water comes out of the spout then your shower head's clogged.
Remove the shower head and try the water. If it comes out of the pipe then take and soak the shower head in CRL or Lime Away for a hour. Now try it. Good luck, Tom

Not broke.. but I bet the shower head is clogged.

Mineral deposits slowly reduce the size of the holes... the occaisional grit or crud that makes its way through the system hits that reduced hole and is more likely to clog it up than pass through it.

Its easier to buy a new head... but you can usually clean and unclog the old one. Soak it in white vinegar or a produce like CLR the dissolved mineral deposits and try to reverse flush it.

If shower head is not blocked, the temperature balancer in shower valve or cartridge may be blocked or may need to be cleaned out or replaced.

Simply remove the shower head and turn the water on. If water comes out, then the shower head is clogged as mentioned above.