Worksheets that stop working

I have been having this problem lately. My worksheets appear as a big blank page with small image icons and all the information disappears. I can no longer use those worksheets so I have to other option than erase them and redo them again. They took a lot of work to do, and many times I had to reschedule activities or create materials last minute because of this issue. Could you help me with a solution? or telling why this is happening so that I can avoid the issue next time? Thank you.