In order to repair a trunk that won't open, you will need the following: a screw driver.
You will need to climb into your trunk, through the back seat. Remove the back seat. Pull the emergency release. Remove the trunk lock. Unscrew it. Look for anomalies. Bent screws could be the cause. You will need to repair any broken parts or replace them, if you decide not to replace the entire lock. Reinstall the lock in the trunk.
You will want to test the mechanism thoroughly, before you close the trunk again. Don't close the trunk, until you are certain that it will work. Otherwise, you'll have to get in the trunk again, by removing the back seat.