Issue with png Textures

I am working on a pretty large, and occasionally my textures stop working. I am using pngs to do image texture mapping in with Cycles in blender 2.70 and I have no idea why all of a sudden these images go black? I have an iamge showing what I see in the texture drop down. The thumbnail is black and the image does not appear in the text editor.

Any suggestions?

Nick Butler

Save blend file and OPEN it again. That should clear lists and traces to data blocks with no users (“0” in front of the file name).
Save one more time.