Wallet app

If it disappeared from your Home screen you must have accidentally deleted it. You can try force restarting your phone:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky.

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But if that doesn’t fix it go to the App Library (the last page of the Home screen) and drag it back to where you want it.

You can check if it is in your App Library and put it back on your Home Screen.

Find your apps in App Library on iPhone - Apple Support