I have a Stanadyne single tub/shower faucet in the guest bathroom which has not been used for several months. Only hot water comes out now. I've read the comment of removing the cartridge. I've removed the copper retainer clip, replaced the handle, pulled as hard as I can, but it won't budge. Any further advice?

This is a Moen product. Moen makes a cartridge puller, (see image) for those stubborn cartridges. If you've removed the copper clip then take the hadle off and use the puller to remove the cartridge. You can pick up the cartridge puller at most plumbing supply houses. Check the inlet ports for any trash and then place a pan over the open valve and turn the water back on to flush out the supplies. Turn the cold watere on first and check for a strong stream,
Reassemble and test. Good luck, Tom