I have a jaguar xj8 and the gear got stuck in park ,and I can't put it in reverse or drive. How can I fix this problem

Most likely the lock out switch at the brake pedal is at fault, because in order to put it in in to any gear you have to step on the brake first ! Their is a switch located which prevents you put it in gear with out stepping on the brake pedal, so check this switch first.

First - make sure you have automatic transmission fluid (check the level it's probably low). Second, on automatic transmissions (I'm assuming auto by your wording of the problem) that require the car to be in park to start etc there is almost always a manual way to disengage the shifter. Usually this looks like a small plastic tab near the shifter gate, pop it out carefully and there's a button under it. This is all in the owner's manual. If it's stuck in reverse you may also attempt to push the car forward a bit or have someone rock it a bit while you try to disengauge it.

I own a 2004 XJ8 that was just towed. The Jaguar has an electric parking brake that automatically engages when the car is in Park. To release the brake, insert the key and turn to start. Then push the parking brake lever down, then the car should roll. Neither one of the tow drivers knew this and the manual does not tell you this. I do not know if this works with other Jaguar models. Good Luck.