Excel 2007 : Work Book in Small window

Hi guys

Wondering if anyone can help.

Have been working on a large file for some days now. All of a sudden when i reopened it to continue working on it the worksheet is in a really small window and i cannot seem to make it full size again. It will not let me do anything to change it.

Any ideas as I have lost a lot of work if i cant fix it?

Thanks

Andy