Often, restarting your computer will fix most problems and errors. However, if this does not work, you can try repairing your software if you're using a Windows computer.
1. Begin by typing Control Panel in the search box in the lower left corner of your computer, next to the Start button. Go to Programs and Features.
2. Scroll down the list of programs to Microsoft Office Professional - the instructions are the same no matter what version of Office you have.
3. Click once on Office to highlight it - once it is selected, you'll see that you have a set of buttons at the top of the list. Click on Change.
4. You can first try the quick repair to see if that resolves your issue. If it doesn't, try the online repair.
Note: This will repair the entire Office suite even if it's just one application you want to repair such as Word or Excel.