What operating system are you using, Windows version or Mac OS X version? How old is your computer? Could be the progress bar isn't freezing and it is just taking a long time to build the previews.
it's the latest OS X software. I have tried waiting but it the progress bar would still not move. In addition this problem has occurred to all the photos that used to have previews.
Close all applications, shutdown your system and restart it. How many image files were you trying to import?