Installing Lr will have created a new empty catalog so you need to replace that with your last working backup.
The default location is normally Pictures or My Pictures unless you have chosen a different location.
Quit LR and find the catalog folder and the file with the ending .lrcat for example MyCatalog.lrcat
Rather than deleting simply rename as MyCatalog_OLD.lrcat
Now copy or drag into your catalog folder the .lrcat file from your backup folder and re-launch LR or simply double-click the restored .lrcat file to launch.
You will have to get the actual image files into the same file & folder paths, but it is easy to re-connect whole folders of missing photos if necessary.
See this useful document (fig 8) and the use of Update Folder Location.
Once everything is working OK you can delete MyCatalog_OLD