This is probably one of the most frequently asked questions here in on the forum, so I thought I'd make a short guide to address it...
Chances are, you've lost the write-pin while you were changing the belt. It's a small metal pin that senses the position of the write-protection tab on the inserted disk and if it's missing all disks will seem to be write-protected to the computer. But before you go crawling around aimlessly on that patterned carpet, here's what you are actually looking for: