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The first thing to always try is close the program, then right click on the program icon and select Run as Administrator. Just do this one time and it will work or it won't. But it is always worth a shot.

If this does not work, how large is your site? And have you Optimized All Photos? Then closed and reopened the program. If you have a lot of photos that have not been optimized this can eat up a lot of available memory and slow things down. And it is always advised to make sure you keep the original photos in a separate location in case at some point you need the originals at the original resolution.