How have you tried to open the trunk?
Does it open when you:
a) Use the key
b) Use the remote trunk release by the driver's seat
c) Use the keyless remote
If the only scenario it does not open is (b), then you can either have the stopper issue JTod posted, the remote release may be disabled, or the cable is broken. To check if the remote release is disabled, open the trunk. There will be a little tab in the plastic housing, just below the latch, usually orange in color. Flip it, close the trunk and test the remote release again.
If it's not working for any scenario, the cable going directly to the latch could be broken, although this is not common.