Have you accidentally created a new catalog?
From the empty catalog try:
File >> Open Recent
If you are using LR 6/CC 2015 the backups are in compressed format and in the folder where the last back was made (be something like this format 2015-07-26 1635) you will find a file named like this "<catalog name>.lrcat.zip" ... you will need to use a application like 7Zip to extract the lrcat file and then open that.
Many thanks for the solution as I was dreading the thought of importing. 7Zip downloaded and the problem solved.
Thanks for the suggestion.