Where did the flash light disappear to?

Without knowing which iPhone you have and which software version it's running, when the phone is unlocked you can try swiping down from the top right corner of the screen to bring down the control center. As long as the control center hasn't been customized, you should see the flashlight button there.

Normally, it also displays on the lock screen (flashlight is usually on the bottom left and the camera button on the bottom right).

No worries and glad I could help. The lock screen is the screen that shows on the device when you first pick it up or press a button. This assumes that you have enabled a passcode to unlock your phone or are using Face ID to unlock it using biometrics.

Check out this link for a walk through for changing the background/wallpaper of your home and lock screens.

https://support.apple.com/en-us/HT200285