My driver's side rear tire won't turn, what do I do?

So, the tire is entirely locked? Check the parking brake mechanism - see if for some reason one of the adjuster springs or self adjuster has broken inside the brake drum. You could also have a differential / axle shaft drive issue, but very unlikely.

Perhaps the emergency brake cable is not releasing the brake on that side or something inside the rear brakes have come apart (broken spring) the rear will have to be jacked up, the wheel and drum removed and the brakes inspected.