I've lost pressure in my kitchen sink sprayer. The faucet is half on and the sprayer is half on. What's up?

There is a small valve inside that operates the spray. When you open the spray it closes the water off from going out the faucet spout. There could be some rust or trash that is caught on the valve and not letting it close completely. Take the faucet apart and clean it out

Let me add a bit to Letmetellu's advice. This valve is called the diverter and is located in the front part of the valve body after the spout's been removed.
You do not have to remove the cartridge to pull the spout. Good luck, Tom