If you mean - at the bottom of the screen you have lots of grey boxes - and no photos - once the catalogue is open - you might need to show lightroom where the original files are located. Right click on one of the images on the bottom line, and click 'show in explorer' - then it'll give you a message to say it can't find it, but will give you the option to find it manually. Once you've located one image, all the rest should auto-fill as well. (It might be that you have put your original files in another location compared to where they were before you reset your pc)