This sounds like there is debris lodged in the zoom gears. Been to the beach lately?
The gears in the zoom mechanism are very small, and even a tiny spec of sand or other debris can jam them up.
Alternatively the sensor inside the lens that feeds back the position of the lens to the control circuit may be failing - so the camera thinks the lens has retracted all the way and stops.
If the stoppage of the zoom action is abrupt then it is probably debris, if it is a smooth stoppage like the camera is stopping on purpose it is probably a position sensor.
Either way the camera will require repair.
Advance Camera Repair
My C-310 had no XD Card installed when I switched it to macro program - and then everything was blocked - no reaction to even switching it off!!! After taking out the batteries for the first time, waiting a while and re-inserting them, I got a white screen and still no reaction to anything I tried. After a second battery removal, not even a white screen - the camera seems completely dead while the lens is still out :-(