I'm running Lightroom 5.7 on a Windows computer with Windows 7 Home Premium. When I tried to start Lightroom today, it tells me that another application is open so it can't open the Lightroom catalogue. There is nothing else open and I have restarted the machine twice. Help
Sounds like a previous run of Lightroom did not shutdown correctly.
Exit LR if you have it running.
In the same folder as your catalog, there is a file with the ".lock" extension (same name as the catalog, different extension).
Delete the ".lock" file, nothing else.