Was using LR5 when the program froze. Having rebooted I opened LR5 again and was asked all the details as if it was a first install. Somehow the program opened without all the details, but to my horror the Library is completely EMPTY
Look under File->Open Recent
If that doesn't contain your catalog, have your operating system search all of your hard disks for files with names ending with .LRCAT, if you find some double-click on each one to open it.
Thanks dj_paige. Problem solved.