Before calling a plumber, just have a quick look under the sink at the hot and cold pipes going up too the tap, there are sometimes small valves (blue or red toggles that twist) on the pipes, if you have these they can be knocked while moving things around under the sink, just check that if you have them they are fully open.
Do this by running the tap on hot and twisting the toggle each way see what effect it has.
Check the other hot taps around the house to see if they are similarly restricted, if they are then there is something wrong with your hot water system, of not it is something to do with that tap or the pipe leading to it.
If it is only that tap which is affected then check to see if a stop valve is not turned on fully as GTTS suggests. If it is not that it is probably an air lock, which may clear itself if you leave the tap on for a while.
If all taps are affected then it is the boiler or the supply. As the cold water is normal it is unlikely to be the supply pressure, so we can probably discount that, which leaves us with the boiler and the stuff that goes with it. You need to determine whether it is a gravity feed system (i.e. one with a vented header tank somewhere in the attic) or an unvented (pressurised) system (one with a pressure vessel about the size of a football somewhere near the hot water tank). If it does not have a hot water tank then it will be a combi boiler, which is another type of unvented system, but the things to check are the same.
If you determine it is a vented system then all you can do is check there is water in the header tank, you need to find this (somewhere in the attic, it will be about the size of a dustbin) take the cover off (normally a piece of ill fitting plastic or polystyrene) and see if there is water in it. There will be a water supply controlled by a ballcock valve, sometimes these get stuck, so just pull it down a bit and see if water comes out. If there is water in the header tank then there is something wrong with the boiler itself or a remote valve, you will probably need a plumber to fix that.
If it is an unvented system (either with a hot water tank or a combi that heats it directly) then it may be that the internal pressure has dropped too low This happens sometimes and is not abnormal). There will be a pressure gauge somewhere close to the pressure vessel, usually this works best if it is between 1 and 2 bar (about 15 to 30 psi). If it below this range you need to pump it up a bit (you can do this yourself if you have a WRAS loop, which you could borrow from a mate in the pub, or buy from Screwfix for about £10, or you may find there is already one fitted (technically this is illegal but lots of plumbers leave them fitted because it saves having to find one when the system needs topping up).Just fit the loop (there will be obvious screw fittings next to the mains supply and the pressure vessel) and turn on both stop valves until the pressure builds up (it should only take a matter of seconds). Remember to isolate the WRAS loop and disconnect one end to prevent backwards syphoning afterwards. Like an unvented system, if the pressure is normal then there will be something wrong with the boiler and you will probably need a plumber.
So, those are the things you can try (and perhaps fix) before wasting £50 on a visit by the plumber.