If you are using the same deck belt, the same pulleys, on the quills, and no parts fell off, you have problem with how you routed the belt. Basically if it worked when you took it off and it did not work when you put it back on, you have done something wrong.
If don't have the owners manual, you should be able to get one hear. You will model number and serial number:
http://manuals.mtdproducts.com/mtd/public/list.jsp
I think you have put the belt in the wrong side if one the "flat faced" pulleys.
BG