Is it a 318, or 218..?? I looked at the AGCO site, and it doesn't show a 318, but does a 218.
When the pto is engaged, are you getting rotation on the gear box, at the back, or anything moving at all..?? From looking at the exploded view of the 218 drive assy. at the rear, there is a clutch mechanism to engage the apron drive. Could be this kicked out, or a problem with one of the pawl's, that engage the ratchet wheel.
Here is the link to the AGCOPartsBooks:
AGCO Parts Books
Enter as a guest viewer by clicking on "view books".
Then type in 218, in the model number block. Click on the arrow far right where it says All Models. A list of manufacturer's will appear. Scroll down to New Idea, and click on it, then click on search.
Select the 218 manure spreader that appears on the next page that appears.
On the next page that appears will be a list of segments of your spreader. Look/scroll on down to page 20, which show the drive shaft assy., and there on. When you click on one of those segments, to the right, a page will appear with an exploded view of that segment, and a parts listing by name, and part number.
Page 28 shows the pawl/ratchet wheel assy., and the drive clutch assy. in one view.
The exploded view is small, but there is a + & - above the exploded view, near the blue line to enlarge the view. Should give you an idea of what to look for.
I'd guess either the clutch disengaged itself, or you've bent a pawl.