Disc brakes are usually hydraulically operated (rare exception: CB200), and therefore a slightly more complicated and prone-to-failure hydraulic system makes them squeeze. Commonly, they get hung up because there is corrosion around the brake piston, preventing it from retracting when you loosen off the brake. You will need to take off the brake caliper, and clean up any rust or debris that may have accumulated, and give some heavy doses with WD40 or penetrating oil.
Once you do that, re-assemble everything again. With a 8mm or 10mm wrench, very slowly loosen off the bleeder valve. It should slowly seep brake fluid. If you get a spurt of fluid, then the brake is either overheated or has worn components in the master cylinder, and will need servicing. However, this should be enough to loosen the brakes back up for now. Make sure the brake lever is still firm enough to stop the bike before riding home, and that there is not too much WD40 on the rotors to make the brakes slip. Do some low speed tests in a parking lot before getting out on the road.
It is also possible that the problem has nothing to do with the brakes at all, instead it could be the bearings. If your bearings have worn to the point of seizing, usually they will have emitted many miles of whining and screaming to warn you, but if they are seized here is how to replace them:
