You fail to give more details for anything but guesses. Does the key work? Does the lock turn? have you tried pressing hard down on the trunk while you turn the key and then lifting it? More desparate resorts would be using a pry bar at the edges to shift the trunk lid while you use the key--that will take 2 people--you can also damage things-but if you are careful-might help. If you do get it open, of course check the latching mechanism inside on the lid and on the body and fix it before you close it again---Duh!. Some WD 40 may help-inject some WD also inside the key slot. Do NOT use graphite in the key slot-it just gunks up with time-moisture-etc. I am also not sure if you could remove the back seat and access the trunk that way with a flashlight in hand. On the other hand, if somone climbed inside and is holding it down, try luring them out with a hot pizza or something like that. Good luck.