Not sure what could cause this to stop working, however can you try recreating the API key without any restrictions to the domain to see if this helps?
So something very odd is happening. I also cannot just "delete" the API. It is in my clients name and billing is attached. I would need to start all over which does not make sense.
In the API it is showing errors in the graph.
In the website it seems to work on the front end with no developer message but the search does not work for address search. It does work if you click on "Use my location"
You can see on our page if you enter address "1600 Midtown Avenue, Mount Pleasant, SC, USA" then search it still shows all locations
If I do a "Use my location" search by coordinates it does work. Use the Coordinates : 32.833535999999995, -79.8408704 in search and you will it work and display one location that is within 5 miles.
So I enabled the following additional APIs that are not described in your documentation and it worked. I am unsure if I need all of them but working now.
Maps JavaScript API
Places API
Geocoding API
Distance Matrix API
Geolocation API
Maps Embed API
Maps Static API