As we have no idea what version of Office for Mac was installed and what version of Mac OS you upgraded from to get to Monterey your question is difficult to answer.
However, on Monterey it only runs 64bit apps! Office for Mac 2016 (the 64 bit version) or any later version of Office for Mac will work on Monterey. I would recommend upgrading to the 2021 version of Office for Mac or subscribe to Office 365 if you want to stay with Office for Mac. If you are open to alternatives you can use the free version of Libreoffice. All of your old documents are just fine so do not worry about them and will work with the 2021 version of Office for Mac or Office 365 or Libreoffice or the Pages app which is already installed on your computer.