I had the same issue. My app runs 24/7, and at March 13th, 2021 around 1PM GMT-0600 your issue started occuring. My app could no longer connect to a Database.
The following 3 steps solved the issue (for me):
Add this environment variable in Heroku under config vars:NODE_TLS_REJECT_UNAUTHORIZED=0
.
E.G:

Make sure you have SSL rejectUnauthorized
to false
E.G (in Node.js):
const client = new Client({connectionString: databaseConnectionString, ssl: { rejectUnauthorized: false }});
await client.connect();
Add this to the database connection string, Not to the actual environment variable: ?sslmode=require
E.G: (In Node.js)
const databaseConnectionString= process.env.DATABASE_URL + "?sslmode=require";
new Client({connectionString: databaseConnectionString ... });
Reference 1, heroku says SSL must be used in production & add sslmode=require programatically
Reference 2, after doing the above I was getting 'error self signed certificate', stackoverflow post helped solve the issue by adding the environment variable