Try re-synchronising the key.
Key in ignition, turn ignition on ( dash lights on, dont start engine).
Press lock button on key fob. Doors should lock then unlock. You may have to push the button a few times.
If it loses sync again you may need a new fob battery.
The fob battery connections can sometimes break causing loss of sync, check the soldered joints.
Do you have a spare key, if so does that work?
vaux
November 2009