If you are totally at loss where the corrupted catalog may be, Alan's suggestion is worthwhile. However, I suspect the corrupted catalog should be where I indicated, if you never moved it from its default position any time back. I suggest you check there first. If so, remove it and copy the good one from your back-up drive into the same place where the corrupted one was. I believe if you then re-point your catalog to that location once from within LR, LR should always go there for it instead of to the backup.