Images not showing

I can't view any of my images any more, for some reason. They are there, as  the filename shows in the top right hand corner of the box. I've tried going to my Lightroom folder and deleting the "Previews" and "Root-pixels" files (as suggested in another thread), but I get "This action can't be completed because the file is open in Lightoom 64-bit", even after I have closed Lightroom. I have also tried optimising the library. My hard drive is only a quarter full. Can anybody suggest anything?

Thanks.