I dont know what I just did, I may have been holding down CTRL as I did it, but basically, in the layers window, you know normally by each Layer name, there is a small thumbnail of what it looks like? Well when I pressed "something" it made all the thumbnail images just turn to a paint brush, Im sure there is a simple way to get back to the thumbnails but I really dont know how! If anyone could help Id greatly appreciate it.
Try right clicking in the layers palet window but not on any specific layer (the grey area under the 'background' layer will work). You can then choose the size of the thumbnail. My guess is that it is currently set to 'none'.