Yesterday all was well, some downloads were almost or halfway complete, today the progress column shows 0% for everything. Files that were nearly complete are starting over from 0%. In Incomplete Files library, all titles are red but it shows they've partly downloaded and I can launch to preview.

Mac 10.4.6, Limewire 4.10.3 Pro, Java 1.5.0_06

Exact same problem here, except some of my files are red and complete (or basically complete, like 44 out of 45 minutes of a video - who needs the credits?) but the progress is 0% or 12%. Some of my files, one in particular is 700 MB, I had 84% complete and now it's 0% and in incomplete files, its 0MB.
Same as above but not pro.
Mac 10.4.6, Limewire 4.10.3, Java 1.5.0_06

It may be related to this: Downlds refuse to reload (click on link)

For Mac OSX you should run the following program to backup your LW downld preferences: utility that backs up lw's downloads.dat/bak files automatically for mac os x here

Files in your library that are red (excluding your incompletes) are files that are no longer being shared. You can control-click them & choose to share again.

Incomplete files that have turned red have lost their relationship with their partner in the download window. You can resume them. But check to make sure you haven't already downlded other copies of the same file. Sometimes a downld may create more than one incomplete file.

the incomplete files (T-14....-filename etc) in my Incomplete folder? I have to start over?

Well for the video missing the credits, you could cancel it from your downld window & remove it from the incomplete folder.

For the others, perhaps you will need to re-start them by using the resume. And yes the resume function will probably mean they'll start from zero %.

You may want to consider that backup utility program for LW for the future so it doesn't happen again.