What did you do?
Are all of your photos still present in Lightroom? Or are some of them no longer present?
It might be a possibility that you have opened another Catalog.
Please check File > Open Recent and open the other Catalog.
Also do you still have all the other images in Lightroom ?
If your Collection panel has disappeared, right-click (Ctrl-click on Mac) on any header on the left side of the Library Module and turn it back on in the context menu that will come up.