Boxster door lock not working....?

[PBoxstar]

Hi,

I recently purchased a 99 Boxster 2.5L convertible and unfortunately forgot to check if the key actually opens/closes the door... The previous owner insists that the key worked fine when when he had used it a couple of weeks back but now the mechanical door key won't lock/unlock the door. It won't even move a tiny bit to the left or right.
Is it possible that something is wrong with the the tumbler? I don't really know why the ignition works perfectly but the door lock doesn't budge at all...
Any ideas what could be wrong?
Thanks!