The wheel won't roll at all, or is just hard to turn? Also, is the tire rubbing the frame? Sometimes the wheel goes out of alignment and will contact the frame. That's controlled by the two axle nuts, if one came loose the wheel will move out of place. If the wheel is lined up right and won't move at all, then it's probably what Uncle stretch said. Does the brake arm have the little clip hooked up to the frame? Without it, the arm can swing around and tighten up the hub.
Was the wheel working fine very recently? They usually don't seize up overnight. And they usually have enough grease inside to keep from rusting up.
The only time I had a wheel lock up was an old well worn coaster hub that would lock up when I hit the brakes hard. I had to kick the pedal to get it going again, but that was just a worn out hub.