When this happens I delete the preview folder and then Lightroom starts again.
The big problem is that this is now happening once a week. So I spend more time trying to generate previews, than to use the program.
Can someone come up with a general/permanent solution?
I'm using Lightroom 5.
My guess is that your hard drive is starting to fail.
I would run some diagnostics on it, and if possible, put the catalog file and previews on a different hard drive.