using key, turn it and maybe lift the boot after u turn the key?? else sit on the boot or apply some force down then try the key at the same time? then get off the boot?? maybe u have got something stuck between the latch.. like a cloth or carpet etc... or maybe the latch bolts are loose.. good luck man...
can u go into the boot from inside the car?
else, break the lock .. if no luck...

i got a aftermarket key lock thing here that i want to get rid off.
..Keith..