The HTTP 503 Service Unavailable error means that a website can't be reached at the moment because the server is not ready to handle the request. This could happen because it's too busy, under maintenance, or something else which requires a deeper analysis.

How to Resolve the 503 Error

1. Check Resource Usage. The resources that a server uses are RAM, CPU, I/O, entry processes, and website inodes. 
2. Check for Ongoing Maintenance. 
3. Stop Running Processes. 
4. Reset Firewall. 
5. Check Server Logs and Fix the Code. 
6. Restart Your Server and Networking Equipment. 
7. Check Your DNS.