Hot water pressure
The insides of old water lines get caked with "gunk" for lack of a better word, and often you drain your system the gunk will settle and fall down the lines and form a more solid build up. When you turn the water back on, the rushing water will push the gunk through the lines until either it comes out somewhere, or it clogs, most often in a fixture. (although i have seen it plug up an entire 1/2" copper 90)
If someone is somewhat handy:
1) buy a 3/8" braided supply tube or a 3/8" braided dishwasher connector (same thing as supply tube but about 5' long and bit more money)
2) and have them shut off the water at each of the offending fixtures.
3) Disconnect the water supply tube AT the shut-off and attach the recently purchased supply hose.
4) Put other end of the tube in a bucket, or in the fixture itself, and turn the water back on and see if you get any pressure.
If you do, then you have a clog in the fixture itself... probably gonna want to call a plumber.
If you don't, then the clog is most likely in your water distribution system.
So at that point you could:
1) shut off the hot water at the offending fixture
2) shut the water down at your HWT
3) open the drain port at the bottom of the tank (might need a hose)
4) disconnect the cold supply at the offending fixture and use your braided supply tube to connect both the hot and cold shut offs
5) open the fixture's hot shut-off... nothing should happen since the hot water was shut off @ the tank
6) open the fixture's cold shut-off and hopefully the cold water pressure will be enough to fire the blockage back down the hot line and out the thru the drain port @ the tank.
If that doesn't work, call a plumber.