That is a good sign
, indicating that the more catastrophic possibilities (engine internal rotating parts damage) have not occurred in your case.
My quick thoughts for failure possibilities to explain your current situation of the rear camshaft sprocket not turning the IP belt, in order of best case scenario to worst:
1. Loose IP belt. Not very likely given that everything has been working for some time. No way really for the belt to loosen. If the IP mounting came loose, it would drop under its own weight which would tighten the belt.
2. Old/worn IP belt deterioriated to the point of losing enough teeth to not engage with the rear camshaft sprocket cogs.
3. Seized IP
Can you test the IP belt tension? An easy method: In the middle, between the two sprockets, twist the belt between thumb and finger. Belt should be able to easily twist at least 45deg to almost 90 deg without a lot of force. Any tighter that that is too tight, any looser is too loose.
Do you know how old the IP belt is? Does it look somewhat new and fresh, with sharp edges and squared-off teeth or is it weathered, with smoothed and rounded edges, and cracking like old tires?