Are you unable to use an Office application? The main reason behind this could be the expiration of the subscription. But, if the subscription is active or not expired and you are not able to open Excel, Word or any other application then you need to repair it using the right procedure. Fortunately, you can do this on your own by following the instructions given below:

Right-click on the Start button
Choose apps and features and search for Office 365 (complete package) app
Click modify and then choose Online Repair
Hit Repair
This will automatically repair the issues and make your Office 365 subscription (available at office.com/setup) run like new!

(Please note that the aforementioned steps for repairing are valid only on Windows 10. If are using any other version of Windows operating system then it is recommended to contact the Microsoft support services and get an immediate assistance from an expert.)