Download, wish and failed lists are stored in "DownloadsV2.db3". It's a file you might want to backup from time to time if you keep a bunch in there.
Once the file is damaged, it's not easily repaired. I'd say it's mostly impossible to recover. The only thing that might save you is if somehow or other the data folder changed so, it's not reading the correct file. I'd search around your disk and see if you have multiple copies of that file.