I thought what happened on my 2004 156 SW might be of interest. The tailgate refused to open. Tugging repeatedly on the cable it eventually opened. I found that a small bolt (10mm AF) had dropped into the lower part of the lock mechanism, preventing it working. Presumably the repeated tugging on the cable had (luckily) shaken it out of the way. I could think of nothing that I'd had in the boot that such a bolt could have come from, nor could I see anywhere in the boot area of the car where the mystery bolt might have come from. Then I pushed and pulled on the upper part of the lock mechanism (the bit attached to the tailgate), and found it could be pivoted. I wasn't sure whether or not it might have been designed to do that, but I removed the small plastic cover and, peering up with a torch revealed a threaded hole from which the bolt had fallen out - the mechanism was pivoting on the one remaining bolt. Mystery bolt screwed back in, the other bolt tightened, everything that moved WD4O'd, job done with much relief. All of which to suggest that you check to see whether the upper part of your lock mechanism feels slightly loose - before this happens to you!
Hope this helps someone,