I have Lightroom 5 on my PC. Everytime I try to open it, it says " The Lightroom catalog named 'Lightroom 5 Catalog' cannot be opened because another application already has it opened". What does this mean? And how do I fix it? it is not open anywhere else on my computer.
It sounds like you had an abnormal shutdown that left a lock file behind.
Navigate to the folder where your catalog is stored.
There will be a file yourcatalogname.lock in that folder. Delete only the LOCK file and your catalog should open fine again.