I unlocked the truck with the key, not the button and now it does nothing but flash the dash board lights.

starter neutral safety switch try shifting car in neutral and back in park

anti theif problem. try un hooking battery for a few to reset computer

I had a problem where my key fob did not lock or unlock the car, Renault Clio Dynamique, but the central locking button in the car did still work and engine turned over fine. Changed the key fob battery still nothing. Read online about disconnecting and reconnecting the car battery, (negative only) and hey presto it worked! All working fine now!

Insert key into door barrel or ignition lock and turn key, whilst doing this press the unlock/lock button, the led on the dash should flash to indicate this has initialised the key to the car, then remove key and test operation of remote. If this fails then a new remote plus coding to the car by your dealership is needed

If it is the anti theft system you can reset it by unlocking the passenger door with the key.

Try re-programming the remotes, here is the procedure for your car.
JAGUAR
XJ8 Remote Control Programming
1. Unlock the trunk with key.
2. Turn Ignition on.
3. Warning light on dash comes on.
4. When they go out, lift arm rest, inside their is a valley button, Press 5 times within 5 secs of lights on dash going out, Horn & Lights Flash.
5. Press any button on remote, remote programmed

first make sure,that the batter is ok. You will need to have both remotes while doing this.
1: get in and lock the doors with the power door lock
2: insert the key into the ignitoin repeatedly IN and OUT 6 times in a row or just keep doing so, until the hazard flashers flash on the dash board (observe the dash board while inserting key)
3: after the hazadrs flash TURN key into the ACC position/not start.(dash will light up) DO NOT START
4: with the key in ACC position, hit lock button on the remote. That will program that remote.
5: to program the other remote simply unlock the door with the door lock switch and relock it with the swithc and hit the other remote button.
6: done

" turn the key on engine off 8 times. the doors will lock and unlock. push one of the buttons on the key fob and the doors will lock and unlock. do this for each remote that you have. when you are done turn the key off. the door locks will lock and unlock. the remote fob should work after this."
From My SecuriLock..

You need to program the remote. Get in to the car and LOCK all doors with the switch on drivers side door. Take the key and insert it into the ignition cylinder in and out 6-10 times, until the flashers/hazards flash on your dash board. Or just keep putting the key in and out (nice and easy now, not fast) until you see the lights on dash flash. After the lights flash, turn the key into ACC mode to make the whole dash light up.(do not start car) .With the key in ACC position, hit any button on remote, that will make the hazards flash again. Turn key off and unlock door with switch again. Open door.DONE.
Good luck.