It might help to know the exact error, if what i say doesn't help. If you haven't already, in the Servers tab click on the Log tab to find the errors.
Usually this means the xxx.part.met is corrupted, the error message will be something like "Error: Invalid part.met". eMule makes a xxx.part.met.bak automatically, on restart it usually tries to automatically use the .bak to replace a corrupted xxx.part.met but if this didn't work you can try renaming it yourself. (Shut down eMule, delete xxx.part.met, rename xxx.part.met.bak, retstart eMule).
If this doesn't work, you need to regenerate the xxx.part.met file. You can do this with metfileregenerator.exe, there's a thread on it here:
http://forum.emule-project.net/index.ph ... pic=134086
The eMule mod I use (MorphXT) has an import feature which is much easier. Some other mods have the same feature, not the official eMule release though. You add the file as a new download, start it and run it until it has gotten the hash values, then pause it and import the old xxx.part file.
If you can't remember the ed2k link, check the "Source Lists" subdirectory, the link for each file is in there. If that doesn't work, and you had eMule set to create .part files full-sized (recommended) and you got it from FLM, you can search FLM for the filesize (in bytes, no commas) and it will show you posts on FLM with ed2k links for files that size.