Excel spreadsheet not responding on my mac

It would be helpful if you provided us with some contact or more specifics. For example, has the operating system been upgraded? I am going to guess that you will have to upgrade your version of Office and install a newer version of Excel.

You say "My new Mac Pro has lots of space", but you provide no details so I have to assume that you are incorrect and believe what the Mac is reporting. It may be time to get an external disk so you can copy files onto it and then delete them from your hard drive to create needed space.