Some students have experienced "scratch disk" problems. This usually occurs when dealing with large or with multiple simultaneous files, or towards the end of a session where you've tried to deal with many images and/or operations.
When you enter Photoshop, it sets up a workspace called a "scratch disk". It uses this to remember a bunch of things that you previously did. For example: if you want to "undo" something, if you have reduced an image and want to return to the original, if you have moved from one colorspace to another and want to return to the original, if you want to go back to an image you recently closed or rotated, etc. I'm not at all clear on how it decides when to let go of things in the scratch area.
Here are some techniques that might work the next time you encounter this problem; try any one (or if necessary all) of them: