A family of Microsoft word processing software products for creating web, email, and print documents.
Is the problem document-specific? In that case, the document may contain a protected content control.
On the other hand, if this happens with any document, either your Office installation is damaged or you are running an expired trial version. You can try repairing the installation by starting setup from Control Panel (the details depend on which operating system you are running).
If you need a new version, you have to purchase one.