My iPhone 7 suddenly will NOT allow me to open the wifi.

Have you checked Settings/Screen Time for any restrictions?

First is to try Reset Network Settings at Settings > General > Reset.

Second is to backup then update the iPhone, if not already on 14.2, using a computer. See "Update your device using your computer" at https://support.apple.com/en-us/HT204204

Lastly, if still grey after step 1 and 2 above, is to restore to factory settings using a computer following https://support.apple.com/en-us/HT201252

If it's still grey after this that means it's a hardware failure and needs repairs.