I do a lot of photo and image editing on my Mac, and while I use Gimp for the heavy lifting items, it's often easier to use the built-in Preview App that comes with all Macs.
After having Preview open for a while and saving screen shots or making quick edits to an image I copied from the web or from a document, I can easily have 25 or more image windows open in Preview.
When that happens, Preview gets all Passive Aggressive and decides it's not going to play the game any more... which you'll notice the next time you try to open Preview - it simply freezes up and gives you the spinning wheel, which is it's version of a 3-fingered solute ;)
Getting Preview to open back up without freezing is a 2 step process.
1. First you'll need to Force Quit the Preview App, because since it's frozen you won't be able to access it's menus to close it down. You can Force Quit the Preview App by clicking on the Apple icon in the top left of your screen, clicking "Force Quit" from the dropdown menu, and choosing Preview from the list of running apps.
2. Next, open a Terminal window. You can find the Terminal App in Finder, under Applications --> Utilities.
3. Type in the following commands, to change to Preview's library folder and delete the contents of the folder.
cd ~/Library/Saved\ Application\ State
rm -rf *
After that you should be able to open Preview. It will still open all the image windows for each image you were working with, but it won't be frozen any longer.
Be sure to close all of those windows before shutting down Preview, otherwise you'll have to run through these steps all over again.