If the latch mechanism won't turn with a screwdriver either the latch is stuck or the lock mechanism is stuck. You might try flooding the lock with Blaster for a couple days and try it again with the screwdriver.
As has already been said unbolting it will also work, it's easier to just turn the latch with a screwdriver when everything is operating properly and just release it and not have to re align the latch, in this scenario it may be the only way to get it open. Drilling the lock out would be my last option, if it won't free up with soaking it with Blaster and the bolts aren't accessible it may be the only way. Lotsa luck.................
