Thanks again everyone for your kind words. This morning when I got up Clyde was at the back door. I nearly fainted with relief. He smells quite strongly of mildew, so I think he might have been in someone's basement or shed. I don't think I have ever, ever been this relieved.
Clyde had his favorite breakfast, and has been asleep ever since. I never want to feel so helpless again! I will be extra careful when opening the front door now.
Once again, thanks so much for the kind posts, I believe that the thoughts and prayers are why Clyde is home!