Come summer, I find out the AC doesn't work?

Hi, I recently purchased a Jeep grand cherokee this winter, and come summer, I find out the AC doesn't work. I recently hooked it up to a pressure gauge, and it immediately rises into the red (over pressure area), then a click, (must be a safety switch), goes off, then the pressure drops, and rises again, repeating itself till I shut it off. Any advise would be greatly appreciated. Thank you in advance.