Workbook Mysteriously Changes Content

I have four workbooks (e.g. A, B, C, D). Everything has been fine up until now. Today I tried to open "A" and it contained only the worksheets associated with workbook "B". The original worksheets for "A" are GONE! Workbook "B" contains "B" content and "C" and "D" also contain only their normal content. I tried to use the version feature and all the versions are from yesterday. I viewed the first two at the top and they are the same WRONG CONTENT. I then went to the oldest version from yesterday and it and others just "hang" when I try to view them. I hope I didn't lose my work completely! Please help.

Thanks!

Ann