Is it the flashlight symbol that has disappeared from your quick settings or are you not able to activate the flashlight using the quick settings?
If the symbol has disappeared from the quick settings, try adding it again by tapping the pen icon in the bottom corner of the quick settings followed by dragging the flashlight symbol to the quick settings.
If you are not able to activate the flashlight from the quick settings, try clearing data for the camera app This will reset all settings for the camera app and the flashlight which might do the trick. Find and tap Settings > Apps & notifications > See all XX apps > Camera > Storage > Clear data > Tap "OK" > Restart the phone.
Try these measures and let me know how it goes! And please return if I have misinterpreted your question in any way.