I would suspect a fault in the power connections, the switch, the fuse, the relay and the window motor. If they are all fine, I'd check for a mechanical cause.
If it is both windows, it is unlikely to be the motors that are faulty or a mechanical problem. Each window probably has its own switch so it is unlikely both would have failed at the same time. On balance, I'd look for a blown fuse, faulty relay or disconnected power line.