If you flip the camera over and look at the area where you stick the SD Card in, just above it will be a recessed button with a downward arrow pointing to the button. This is the manual "gimbal" reset button. For it to work, turn on your ST-10 and Q500 as you would normally do, once booted up take a tooth-pick or similar shaped instrument and push in the button until the Q500 beeps at you.
NOW you turn off the Q500 and ST-10 and when you restart it again, you will have manually done a gimbal reset.
It also may behoove you to do a compass calibration again too if the gimbal reset doesn't work. Some tilting gimbal issues have been resolved this way too.
Good luck.