This article explains all the things you need to check and fix when the WiFi signal on your iPhone is grayed out.

Make sure Airplane Mode is turned off. This may sound silly, but before you do anything else, make sure Airplane Mode isn’t turned on. This feature disables Wi-Fi (and cellular networks) because it’s designed to let you use your phone on an airplane, where outgoing wireless communications are often not allowed.

The easiest way to see if Airplane Mode is on is to open Control Center by swiping up from the bottom of the screen (or down from the top-right corner on iPhone X and later). If the airplane icon is active, tap it to toggle Airplane Mode off and your issue should be resolved. If it’s not active, something else is going on and you should move on to the next step.

Update iOS. Although unlikely, the Wi-Fi issue could be due to a bug in the operating system. You can either update your iPhone from the phone itself or use iTunes to download and install the latest version of iOS. Once the update is complete and your iPhone has restarted, check to see if Wi-Fi is working.