Spreadsheets stink

One of my monster spreadsheets started acting funny a few weeks ago. For no reason, the look up function I've used for years stopped working on random rows. I figured out another way to do the look up that worked. Today the whole spreadsheet stopped working, both ways. A friendly co-worker helped me get most of the rows working, but I still had to manually type about 50 rows of 5 columns. 1/2 the day shot. Whee.