Collections have disappeared???

Do you mean that the Collection panel on the left is missing?

If so, use the menu command WIndow->Panels->Collections

Are you asking if the images are in Lightroom somewhere? The images are never in Lightroom. The images are in folders on your computer. The only file that Lightroom opens is the catalog, which is a database. That catalog simply contains a reference to where the images are located as well as a record of all the work you have done to those images. The catalog also is where the collections are created, and those collections only exist within the catalog. In other words, the Lightroom catalog is the file of primary concern. You need to make sure it is routinely backed up. You also should ensure that you have a good backup system for protecting your images. Lightroom itself does not back up your images.