What has changed on your computer? Are you getting an error message?
Have you changed the name or location of your catalog? If not, you should find a file called catalog.pse13db in the following directory on your computer: C:\ProgramData\Adobe\Elements Organizer\Catalogs\My Computer\ .
If you changed the name of the catalog, the folder name will be the same as your catalog name. Once you find the file, double-click on it and it should open your catalog. (Make sure that File Name Extenstions are visible when searching in Windows - there is a box to check in the File Explorer>View tab.)
If you have moved the catalog folder location, search your computer for the catalog file named above.