Your website has exceeded the free limits for database and the database will have been frozen, after a few hours it normally starts working again after the cool down.
Sadly when your site exceeds the free limits there is nothing we can do apart from recommend an upgrade sadly.
You can also go to alternative free hosting too.
So if you see the topic above it details the limits and confines of the free plan.
It also offers the plugin of
If you install that (when you can)
Then DISABLE all the heartbeat functions (not the plugin itself)
You can also use
It will give you a rough idea of how many requests you are using in the backend just navigating around wp-admin and the front end too.
So if you use 50 requests loading a theme page in the backend, 5000/50 for example gives you a limit of 100 pages loadable in an hour in the backend before you hit rate limiting again (only an example)