First, in order to fix the issue, I suggest you use the Repair feature. Go to Start > Control Panel > Unistall a program, select the software from the list and press the Repair button next to Uninstall. If it doesn't solve the Microsoft Word problem, remove the utility from your computer and reboot the PC. After that, if you're using Microsoft Office 2007, navigate to Microsoft Office Download Center, select the application version, enter the Product Key and follow the provided steps. Do the same if you're using Microsoft Office 2010 or 2013. If you no longer have the serial number use the guide provided by Microsoft to recover it. Alternatively, before removing Microsoft Office from your computer, download and install Magical Jelly Bean Keyfinder. After the installation process is complete, launch the software and it will find the product keys for all installed products.