Get QuickPar. If PAR2 repair files are posted, download the small one and open it in QuickPar, which will tell you how many repair blocks you need. Download those and repair the corrupted files. If there are not enough repair blocks available, you can check RAR files with WinRAR to see if they have recovery records. If they do, repair all corrupted ones with WinRAR first, then run QuickPar on the archive set.