The simplest thing would be to check the shut off under the sink itself. It may have accidently been turned off, or was shut off for another purpose. If you are getting water but not hot you will probably need a plumber as the line is connected to the cold water supply.
First, make sure the hot water valve under the sink is turned on. If the hot valve is open and you still are not getting hot water to the sink, you may have a clog in the flex pipe running to the faucet. Or, the mixer in the faucet itself may need replacing.
the hot water may not be working because the line could be stoped up. also the rubber seal inside the handle may be broken and stopping the water from moving.