First try the everyday procedure of soaking it with penetrating
oil and tapping it with a hammer to try to break up whatever it is that is holding the threads. If it is the type that uses a standard key to open, the holes of the key are generally too close together in the center of the cap to transmit much torque, but if you drilled an indent (just a partial hole and not through into the threads) closer to the outside edge of the cap, and used a punch, you could use a hammer on the punch in the indent, to drive the cap in the counterclockwise (unscrewing) direction, and it should loosen up.