Android app API key not working

I made an android application in Java using an android studio that shows a route to a destination and the points of interest along the route. the application was working fine until the google maps API key subscription ended, now I have purchased a new key and some of the features of the app have stopped working even though none of the code was changed. I need help finding the solution. all the code is done just need to know why it would not show the route and the points of interest like before. its urgent