I have lived in this house for approx. 2 years. My Kitchen sink has never had hot water. I checked the hot water heater , and that is fine. All my other sinks, showers, dishwasher have hot water. The water heater is in the laundry room, which also gets hot water.

It is an old house and all the pipes are under the house. I believe that there is only one main pipe, but I can not figure that out.

Please help.
Thanks

Does your faucet have two handles? If it does and you turn the left handle to the on position does water come out? Even if it is cold water? Or if it is a single handle faucet turn it to the left as before and I ask the same questions.
In no water comes out you have a pipe stopped up or there is a blockage in the faucet itself. If you do get cold water out then I would think that your faucet is connected to the cold water line twice.

Thanks Letmetellu.

My faucet is not a knob, it is one of those that you push up and down to turn on/off.
The water comes out fine, and I have checked under the sink by turning off the hot water and cold water pipes, which seem to be fine. So if they are both connected to the same cold pipe, would the hot water not turn off? How do I check?

I think it would not be a safe assumption that pipes are crossed because this sink was fine before. I am assuming it is a blockage at the stop cock or more likely the mixer. Shut off the stop cocks under the sink and open the nut on the top of the stop. This will allow you to temporarily remove the supply line to the faucet, just do the hot line. Get a large pan and a towel to catch the water and then open the hot stop cock a crack or two. Is that water hot? If so we can move onto the mixer but I may not be of much help here. When you say "one of those that you push up and down to turn on/off" are you describing a lever type handle, or could you further describe the faucet, pics or brand would help too.

There are several faucets that have a knob that you pull straight out. There is a Kohler, a Briggs, a Cole, a Moen and I am sure there are others that work the same way.
If you turn off both of the supply valves under the sink, you should not have any water coming out the faucet when you turn it on. Then open the supply valve on the left side under the sink, this is usually the hot water wide. Now turn on the faucet, you should get a stream of water that after a few seconds to a few minutes it should get hot. If you do not have a stream of water then you have a blockage some where in the line or faucet.
Now after turning off the left valve under the sink, turn on the right valve under the sink which is normally the cold water pipe. Open the faucet and you should get a stream of water, after running it for a few minutes it should remain cold.

Back to the point where you tried the left valve under the sink, if you did not get a stream of water there is probably a blockage in the faucet and you can check this by turning off both supply valves under the sink, open the faucet, make sure no water is coming out of the faucet, if not then you can take the faucet apart and check for a blockage int the ports of the faucet.