Hi ,
If you can still turn the key it will mean that you will be able to remove the lock from the lock housing, normally you turn the key to position two, look carefully at the lock housing, you should see a small hole in it, about half way down the lock barrels lenght, push something small ( allan key ) into the hole, this should let you now pull out the lock. Most Vauxhalls are like this, the Frontera should be the same, but dont take my word for it.
Reasons for getting stuck
Have you just had a new key cut ? there maybe same burs left on the key.
Its not likely that the waffers in the lock are wore out, as this would stop you turning it & you have not said of any problems with turning the key.
The only problem with having to obtain a new lock & barrel is that you will have it coded to the car, or you will have to try & remove the transponder chip from you old key & swap it for the new one.
Kind Regards
Haydn