Not much to go on here. First thing to to do is see if you can locate your Lightroom catalog on your hard disk. Do a search for any file with the extension .lrcat (search for (*.lrcat). If you do not find one, then the file was not stored on your internal drive and you will have to determine if it could have possibly been store to an external drive, a network-attached drive or even possibly a cloud drive). By default, Lightroom saves catalogs in the following folders:
- Windows: \Users\[user name]\Pictures\Lightroom
- Mac OS: /Users/[user name]/Pictures/Lightroom
By default, the catalog is named Lightroom 5 Catalog.lrcat unless you specified a different name during creation. When you locate the .lrcat file, open it. This will start Lightroom using the selected file as your catalog. You should then see all your photos and previews as expected. In Library mode, check the top level folder displayed in the left panel. You should see a small green rectangle indicating Lightroom has connected to your folders. If not, use the Lightroom locate function to find and reconnect your folders as needed.
Once this is done, go into the Preferences panel and navigate the the General tab and use the drop down box under Default Catalog to select what catalog Lightroom should use. Select the "Load Most Recent Catalog" option which tells Lightroom to use the catalog you are now viewing the next time you start Lightroom. Keep in mind however that, if you have more than one catalog on your HD, Lightroom will always open the catalog you opened the last time you started Lightroom.