I most recently experienced this after mine went very low. Open the flipper door using the top down button but don't retract the top into the box. On the drivers side, under the large black box where the motor is, is a small plastic container with fluid. It has a brass fitting. Remove the fitting and fill with a funnel but DO NOT FILL ALL THE WAY! Good luck! Mine is finally fixed! Freezer mentioned that low hydraulic fluid is a typical cause of "Roof Cycle time OUts". You can get the fluid at any Chevy Dealership or use the SAAB hydraulic fluid that is a substitute. :seeya
Apparently it just gets used up over time. They could not find any leaks that they know of. I checked today and it is just fine. You will see a Max line for the fluid level on the little plastic tank that holds the fluid.