I have been using LR for about a year, I use it daily. I edited some of a session last night, then went to bed. When I woke up this morning and turned on the computer, my last 2 weeks of folders of sessions was gone. Then to make things worse, I restarted and shut down my computer again to see if they would magically reappear and now every time I try to open the catalog with dozens of my recent pictures/folders/sessions in I get this message: "The Lightroom Catalog named "rename" cannot be opened because another app already has it open. Quit the other copy of LR before trying to reload." But I didn't have LR open on any other device, and I don't understand how it could be open in another app and if so where to go to close it...I only have LR on this computer. I can get into lightroom and upload pictures but only because I created a new catalog. I cannot get into the catalog I need desparately to. I already tried resetting my computer to an earlier date which didn't help. Thank you!!!!
Your catalogue has been corrupted. If you have a backup of it, close LR and then copy and paste the backup over the original catalogue. Then open LR
When Lightroom is open it creates a file with the same name as the catalog with an extension .lock (LOCK) alongside the catalog file.
This lock file prevents any other access to the catalog.
If Lightroom is closed normally the .lock file is automatically deleted. However if Lightroom closes unexpectedly e.g. loss of power, crash then the file may remain in place preventing Lightroom from opening the catalog.
With Lightroom closed go to the folder where your Catalog files reside, by default this is in your User > My Pictures> Lightroom. If there is a .lock file present delete the file (do not delete the .lrcat file. See the screen capture.