Exactly what does the window say? Please quote the entire message word-for-word, verbatim, exactly as it appears.
The title of the window is "Restoring Catalog"
It says: The catalog named "Lightroom 5 Catalog" was left in an inconsistent state. Lightroom must restore the catalog before proceeding.
This may take several minutes.
There is a progress bar below that, which does not show any progress. It's just showing the revolving barber shop bar
It"s been doing this for the last 3 hours. Obviously nothing is going to happen there.
I would restore a recent backup of your catalog file.
I am unable to open LR as that restoring catalog window stops LR from opening. Even tried to force restart my iMac.
If you have a recent backup catalog, you can restart your iMac and then make a copy of the backup catalog, put it wherever you want it to go, and then double-click on the backup catalog