Replaced the supply line to hot water. Faucet worked fine then suddenly lost almost all water supply. The hot water line is split and goes to the dishwasher, which also has almost no water supply. Valves are open, both hot and cold. Rest of the house works fine. Am I looking at replacing the shut off valves? What could be blocking both hot and cold water?

Particles that were shaken loose from the work, clogging the filters at the faucet and dishwasher?

Possible. Changed cartridge at faucet. Worked fine for about 30 seconds than water went to a trickle. Did nothing to the cold water, just the hot. Where are filters to dishwasher and faucets located please?

The first thing I would do is open up the faucet, pull the cartridge , place a pan over the open valve and turn the water (hot and cold to flush out the valve body and supply. Check the inlet ports on the cartridge for trash and any debris. Next I would disconnect the hot water supply and flush out that line. Next I would clean the inlet screen at the dishwasher inlet. Reassemble and test. Let me know if that worked for you. Good luck. Tom

Replaced the supply line to hot water.

What kind of pipe?

Assuming copper pipe, you probably used too much solder at a joint when replacing pipe. If you use too much solder it can run inside the pipe and hit a section of cold pipe. Melted solder will not stick to a cold pipe. That drop of solder has now moved to the next available valve and blocked the valve port.

Remove valves, flush hot water line and replace valves.