Can you retrieve a previous recording on VCR tape that has been accidentally recorded over?

No. If a tape is damaged (broken, degraded, dirty, moldy, etc...) it is possible for an experienced professional to repair it. Once a VHS tape is taped over though, the original audio and video is gone. The best you can hope for is to check the very beginning and very end of the tape to see if a couple of frames of the original video are still there.