The problem started very suddenly yesterday and now I don't know how to fix it. I've tried quitting and restarting, as well as restarting my computer and reloading old work. Any help would be great!
Which version of Photoshop are you using?
Which OS version?
What does the PSErrorLog.txt file (in your Photoshop preferences folder) say?
Thanks Chris, but I solved it by resetting my tool preferences and clearing away some damaged fonts!!