There are a couple of simple steps to follow to make sure the notifications turn back on. 1. Go to your iPhone and open Watch settings. Tap on the Notifications and turn it back on. 2. If that does not work then you can try switching off your Apple Watch by touching the side button, it would be even better if you turn the watch and iPhone both simultaneously and keep it off for a while. Then turn it on and send a message to check if that method worked. 3. Just like we tried on iPhone by turning off DND mode, the same goes with the Apple Watch, just go to the watch, slide its screen up to show notification options, and make sure the Moon symbol is not on, that is the do not disturb mode. 4. Check that the bell icon is also not selected on the watch.