It might be in there really tight...
If you are indeed talking about the ceiling panel down the center then, besides all the steps mentioned by Brett, you may just have a really tight panel. I was dismanteling my entire interior and taking off all of the interior skins and had a dificult time getting the center section out. It is supposed to be a slip fit into the metal channels along the edges of the center such that it can be slid out once you have removed the rivits or screws that are holding it in here and there along the middle area. You are supposed to be able to get the center to bow down some so that one edge will pop out. I found that one thing that helped a lot was to squirt some silicon lubricant into the channel area to loosen things up. I must admit, though that I could not get the center panel off all by itself. I had to take one of the side panels next to it off first. I used a putty knife shaped paint scraper to tap the moulding off of the center panel on one side. What I found was that there were some additional thin strips of aluminum that had been jammed into the same track, perhaps to help hold things in tight. It sure worked. I hope that your center panel is a lot easier to get out than mine. At least you should now have enough information to give it a good try. Unless of course you are talking about some other panel.