Cut off the water under the sink and remove the cartridge and examine it. Maybe you will see what's wrong and you can repair it and re-install it. Probably you should just take the cartridge to a big box store (or to a good plumbing supply) and get a replacement.
That hot/cold designation on the front of the handle screams Price Pfister.
Moen/Delta/Kohler general try to hide that, or not include it, in their design of that style of faucet.