I need some help

I went out to pepboys and got new brake pads for my 99 ford taurus, and i started working on the back brakes, and i got the calipor off the rotor and took a c-clamp to get the piston back in place, but the piston wont even budge, i cant get it to go back into the calipor at all and now i cant even put the new pads on. Can somone please, tell me what they think the problem could be?

The piston on the rear calipers needs to be rotated while pressure is applied to get it to retract. This is due to the mechanical parking brake mechanism being integrated into the hydraulic caliper. There are at least a couple of other posts in this forum with descriptions on what you need to compress the caliper. Do a search.

Most parts stores (not sure about Pep Boys) have a tool loaner program. You should be able to borrow the rear disc brake caliper tool from them.

-Rod