Hard to tell without more info. Could be a fuse, could be the motors, could be a bad switch. They're both up and neither will go down? Probably not a motor if that's the case. Don't go down when lights are turned off but will when center switch is pushed? That'd be the switch most like. Both stay up no matter what? Fuse or wiring issue perhaps.
But even with more info you'll almost certainly get better answers at
Miata.net's forums.