You can just brace the top shock mounts against the ceiling and use a jack to push the loop up. If you have big friends they could act as balast to keep the fram down while you use a jack to muscle the loop up. This part of the frame only holds the rear fender (mudguard) and rear light, it is not very strong in the upand down plane. Look for any cracks on the braces or near the top shock mounts, this would indicate a more serious problem and do take measurements to make sure the frame is not damaged besides the rear loop, if in doubt ask a competent shop to take a look.
Jean