It is possible that you have (accidentally) opened the wrong catalog file. Use File->Open Recent and select each catalog that appears in the list until you find the one with the 60k photos.
I'm using Lightroom CC on an iMac and yesterday backed up my library. My photo library has around 60k+ photographs on an external drive.
I assume you are using two separate drives, correct? Where are the original files located? What application are you using to backup the files?
I opened lightroom today and under folders for Seagate Back Up Plus drive the lightroom catalogue has 9744 photographs and there are almost 50K photographs missing.
Are you Importing the backup image files into LR? I suggest not doing that since you will lose track of the backup files or folders (! mark) if the "original" files or folders are moved or renamed inside LR. This in fact may be what has happened.