We have a 3 year old home with copper plumbing. We had no problems until we recently installed a mud room with a sink. We suddenly lost hot water pressure at the kitchen sink.
All the other faucets throughout the house have pressure, and the cold water pressure at the kitchen sink is fine and both the hot and cold water pressure at the new sink in the mudroom is fine also. Only the hot water on the kitchen sink is affected. There were no changes to the plumbing to the kitchen sink or elsewhere.

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Thank you.

What type of kitchen faucet do you have, single lever or two handle. Where is the mud room relative to the kitchen? Was the piping for the mud room taken from the kitchen area. The most likely thing is that some solder got inside the piping while plumbing the mud room sink and is now blocking the hot water valve in the kitchen sink.

The faucet is a single handle that sits on the counter top separate of the faucet. The mud room is opposite the kitchen... mud room is toward the front of the house, and the kitchen is opposite that... about 25 feet at the back of the house... there is a laundry room between the kitchen and the new mud room... the water lines were ran off the main lines close to where the kitchen lines take off at... Thanks

As stated, you have something blocking the hot water line, probably solder. It could be at the stop valve of the kitchen hot or at the faucet valve itself. Turn off water, remove kitchen hot stop valve underneath the sink. It is probably a compression fitting an should be easily removed. Look in the input side port and make sure it is clear. If you cannot be sure that the valve is clear just replace it only about 4$.
If cleaning or replacing the stop valve does not work you should next back flush the faucet valve. Get a good size bucket, remove the hot water supply tube from the stop valve and direct it into the bucket. Have someone turn on the faucet in the mid temperature range. You should get water out the hot supply from the faucet. If you have one of those flexible spouts with a spray option, turn it to spray. Hopefully this will flush out whatever is blocking the valve. If not the valve will have to be disassembled and cleaned.