Hi--we had a leak in a hot water pipe in our basement--after the pipe was replaced we have lost our hot water pressure. The pipe is more narrow than the pipe that was originally there, could that be the problem?

A slightly smaller pipe has a huge effect on pressure drop. For instance replace a common 3/4'' ID pipe with a 1/2'' one, and the pressure drop is 8 times as much. The new pipe could be thinner walled than the old and still about the same inside.

More likely, the pipe work knocked some crud loose that traveled to the first fitting or valve and clogged the pipe there. If the pressure is down all over the house, the clog should be between the hot water tank and the first tee. Can you shut the outlet to the hot water tank off, and take the pipe to it loose? Do it, and go to a mixing faucet, turn on both the hot and cold, and hold your hand over the outlet and see if you can force cold water back the hot line and push the clog out. If you can't do that, can you shut the inlet off and open the drain valve at the bottom of the hot water tank? Even shutting all the water off, opening the drain, and a hot faucet might work. Somehow you must make water run backward through the clogged area and out.