Probably a siezed handbrake, often made worse by rusted, weak or missing hand brake return springs.
If you get under the car and look at the back of the wheel you will see the handbrake cable connected to a lever which sticks out of the back of the brake plate. There should be a spring between each lever and the back plate.
With the handbrake off you should be able to pull the lever at the wheel and release the brake temporarily, but it will probably stick again next time you use it.
IF YOU ARE WORKING UNDER THE CAR REMEMBER TO WORK SAFELY.
Use stands to support the weight, do not rely on a jack alone. Chock the wheels securely so the car can not roll forwards or backwards.
If you just want to free it off enough to drive it to a workshop try rocking it back and forward with the handbrake off, it will usually spring off with a bang.
There is a possibility it is the caliper which has siezed, but I would very much doubt it as the car has only been sitting for 3 weeks.
duke
January 2012