Several things such as a prolonged "rich" condition (or a dumb move like I made below) will ruin the CC over time but not cause a blockage. The engine would fail an emission test. Do you have a rattle? I'd put the car on ramps and tap the cat with a rubber mallet The honeycomb ceramic interior of the cat can crack if you hit something ( unlikely) and shift causing an obstruction but as Klaus says, it may idle but not run under load.

In one of my all time bonehead moves, I cross wired our old 740 when changing plug wires (gosh, there were only four...imagine if I'd had 8!) The wife jumped in, fired it up and drove off ignoring that it was running a little "rough". Soon as I heard it, I realized what I'd done but couldn't get her attention. I jump in my truck and chase her down within a mile or so. I pop the hood and the cat is cherry red! Why it didn't catch the whole car on fire is beyond me. I uncross the wires and all is well...for about a month when a terrific rattle developed that I trace to the cat. Pull it off and the honeycomb is in three big chunks. Took a long punch, busted it up and dumped it in the junk barrel. I sure had a "low resistance" cat after that. I'd been screwed if we had emission testing for sure but ran it that way for 7 years til I needed a new exhaust.