If you cannot turn the trunk lock cylinder of your 1998 BMW 318i you could try spraying some graphite lock cylinder lube into the key slot. If this does not work you may need to replace the lock cylinder. I hope you can open your trunk with a release button inside the car or on you key fob. If not there may be an emergency release handle inside the trunk. You should be able to access the trunk by via a rear seat pass through or removing the rear seat.
FIRST .. CHECK THE FUSE THEN TRY UNLOCKING YOUR PASSENGER DOOR AND THEN PUSH THE TRUNK TO SEE IF IT WILL OPEN.... MY 93 DOES NOT HAVE A TRUNK RELEASE(well i never found it) but all the locks should be insync. OH YEA... USE THE MASTER HEY FOR THIS