I suddenly can't use Excel anymore. How can I get it back?

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

Thanks to Matti Haveri…

https://discussions.apple.com/thread/254698315?answerId=258771288022#258771288022

LibreOffice 4.0.6.2_MacOS_PPC works in OS X 10.5.8 (last official PowerPC version).

LibreOffice 4.3.7.2_MacOS_x86.dmg (and x86-64?) works in OS X 10.6.

LibreOffice 7.0.4 works in macOS 10.11 and 10.14.

https://downloadarchive.documentfoundation.org/libreoffice/old/

Ask Microsoft support or use LibreOffice.

Most likely something changed. Did you change the computer's operating system? If so, that could be the cause. Most likely you have to update Office. There may be some costs involved, though.