I'm presuming it's text messages @Janine73? You could try a Manual Roam, which is basically, temporarily, jumping off your network and rejoining, and that'll force a fresh connection and that should kick start your messages, here's a Manual Roam guide here. Obviously choose your own network to reconnect to.
If it's an app the messages have stopped on it could be to do with permissions so please let us know which you're having issues with.
