Just a shot in the dark, because you didn't give much info for what type shower valve, but it sounds like what the guy or gal put in was a new cartridge, and it sounds like maybe it's not referenced or whatever to where OFF is, and where cold and hot are......just a guess but it sounds like you're talking about a Moen PosiTemp maybe.
If you know anything about what you're doing, turn off the water (to the whole house), take it apart (the shower valve), and re install it (the cartridge and handle)....better yet...just get a new cartridge because that one's probably screwed up now.....but if you don't know really, you're going to mess it up and it costs about $700 to put in a new shower valve, as opposed to about $230 to get a plumber out and put a new cartridge in...so if you mess it up, you could really cost yourself alot more money.