It happens .. happened to me a few days ago. Too many incompletes is one of the reasons but may also happen for other reasons. Go to Library window, select incomplete folder, select all incompletes & press Resume button.

BTW I'd recommend Tip: Purge the Downloads Queue! (click on blue link)

With the new version of LW's I'm not sure where the download list is contained. It used to be held in the Downloads.dat file in the incomplete folder. All I can find it the Library file which simply contains shares.

Oh something else, if you have days to keep incompletes set to a lowish number then that will affect it also. I have mine set to 9,999 days or so.