We are sorry to hear you are unable to open a new document in InDesign. We would request you to share the details of your operating system, please, and the version of InDesign installed on your machine. Please also share the screenshot of the greyed-out menu.
Please try the below steps- Launch InDesign > Edit > Preferences > General > select the box that says "Use legacy "File New" interface" > click ok
Then try to open the new document and let us know if that works.