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Thank you @The_wolfes for the quick reply!
In that case, have you tried clearing the data for the app? This will not erase any previously received messages but it will reset all settings for the app which might do the trick. Find and tap Settings > Apps & notifications > Show all XX apps > Messages > Storage > Clear data > Tap "OK" > Restart the phone.
Try this and feel free to keep me posted with the results!
Hi @The_wolfes, let me see if I can help out here.
Is it all notifications that has stopped working or has the notifications stopped working for a specific app?
If it is all notifications, have you checked whether Do Not Disturb (Settings > Sound > Do not disturb) is activated or not? If the notifications does not work for a specific app, please make sure that the notification behaviour for the app (Settings > Apps & notifications > Show all apps > App name > Notifications) is set to either "Make sound" or "Make sound and pop up on screen".
Let me know if that helped and if I have misinterpreted your question in any way, please do elaborate.
It is notifications for SMS that has stopped working.
Do Not Disturb (Settings > Sound > Do not disturb) is not activated.
The notification behaviour for the SMS-app (Settings > Apps & notifications > Show all apps > App name > Notifications) is set to "Make sound and pop up on screen".
Thank you @The_wolfes for the quick reply!
In that case, have you tried clearing the data for the app? This will not erase any previously received messages but it will reset all settings for the app which might do the trick. Find and tap Settings > Apps & notifications > Show all XX apps > Messages > Storage > Clear data > Tap "OK" > Restart the phone.
Try this and feel free to keep me posted with the results!
Tahnk You!
No worries, @The_wolfes! Glad I could help out! 
Let me know if there is anything else on your mind regarding your phone!