I was trying to backup lightroom but the backup was taking longer than 5 hours so I cancelled it. It took another hour to cancel and I needed to continue working on a project so I shut my computer down. Now, when I open lightroom I get an error message "Lightoom encountered an error when reading from it's preview cache and needs to restart. Lightroom will attempt to fix this issue next time it restarts." When I click ok, it shuts down, I reopen it and it says the same error message. I can create and open a new catalog but not that one and I am new to lightroom so didn't realize I could create more than one catalog so ALL OF MY PICTURES ARE GONE!!! Unless someone can help me. Please! Really upset about this and I am halfway through a big project for a client. I do NOT want to have to start over! Thanks for any and all advice!
It sounds like your previews are corrupted, not that your catalog got deleted.
If Previews are the only thing that got corrupted then you can fix things by deleting the previews folder. Its name will be catalogname Previews.lrdata and resides in the same folder as your catalog. Make sure you’re out of LR and delete that .LRDATA folder
DO NOT DELETE THE .LRCAT FILE which is your Catalog file, itself.
Once you have deleted the Previews.LRDATA folder, open LR and see if things are ok. LR may take more time rebuilding new previews for your current working folder and any old folder you switch to in the future but that’s better than nothing working as it is, now. You can build previews for your working folder if you want, using LR / Library: Library / Previews / Rebuild Standard or 1:1 Previews.