I purchased Lightroom 6 a few days ago. It has worked perfectly and now it says "Unexpected error opening up catalog" before even going into the program. It gives me the option to go and select another catalog however, it won't actually let me perform that. So, I can't get in, can't sign out, can't u[load a new catalog. Help please! I'd really like to not lose all my work over the past couple days and use this program!
Look in the folder the catalog is stored in and look for a file that ends with a .LOCK extension. Do this with LR closed. If you find one delete it then try to open LR.