If your brakes are only partially seized, you may be able to release them by rocking the car. Switch the engine on and drive forward and reverse in succession. Make sure you are in a safe position before you try this and stop if you don’t feel an improvement. You may cause other damage otherwise.
If your brakes are completely seized, you will need to get the car towed to a garage for a brake repair.
We would strongly recommend booking an inspection if you notice your brakes become partially seized as well. Continuing to drive on ineffective brakes can endanger you and other road users.