If it is hung up, and hopefully you have auto save set for every 10 minutes, it should recover when you re-boot. I have several like that, one that has to be printed on plotter paper to get the whole thing!!!!
Good luck. I hope the autosave pulls through for you.
Once I was in hysterics because I thought I'd lost a project I didn't save when my laptop crashed, but it was there when I logged back in after reboot. I didn't even know I HAD autosave on that laptop, so you might just be in luck.
Depending on the version you have...
Click on round "Office Button" just to the left of Home.
Click on Excel Options at the bottom of the pull down.
On the left hand side, there is a tool bar showing several options. Click on Save.
On the right, the save options pop up.
The first one listed is "Save Workbooks". Make sure the "Excel Workbook" options is selected to save it in the current format (or adjust accordingly). Make sure the AutoRecover box is checked and select the time interval you want it autosaved. You can also adjust the AutoRecover file location if you like.
It is good to be in teh habit to save anyways as the autorecover is not saving the original file, but rather a backup file in case your file becomes corrupt, Excel or computer fails, etc.