HTTP errors usually come in the form of status codes, which are standard response codes that help you figure out what went wrong when a web page or other online resource doesn’t load correctly.
When you get an HTTP status code, it comes with both the code and an explanation, like “HTTP error 503: The service is unavailable.”
The HTTP 503 error is part of the group of HTTP status codes called “5xx server error.” They usually mean that the server can understand the request for a web page or resource, but it can’t fulfill it for some reason.
When you get HTTP error 503, it usually means that the website’s server is overloaded (temporarily) or too busy, or that something is going on, like scheduled maintenance.
Even though this error happens all the time, there are quick ways to fix it and get the website back online.
Initial repairs
Whether the problem is with the server or your computer, there are some things you can try and check before fixing HTTP error 503: The service can’t be used. You could start by reloading or refreshing the page and typing the URL again in the address bar.
You can also restart your modem, router, and computer or other device. This is especially important if you see the message “service unavailable – DNS failure.” If this doesn’t fix the error 503 DNS problem, choose new DNS servers and change them on your PC or router.
Check with the website itself for help, as they may know about the error 503. This way, they can tell you if it’s a problem for everyone or just you. Sometimes the easiest way to fix this error is to just wait for it to go away.
Close your proxy server
If you use a virtual private network (VPN) or a proxy server, you may need to check that the connection is working as it should. If the proxy server goes down, you might get the message “HTTP error 503: The service is unavailable.”
This usually happens with free proxy servers, but if you don’t use one, you can turn it off and then try to open the website that says “HTTP error 503: the service is unavailable.”
Start the destination application pool
Type Turn Windows features on and off in your Windows search bar and select Open.
Find Internet Information Services and select the box next to it.
Press OK.
Press Win + R to open Run dialog.
Type control and press Enter.
Ensure that the View by is set to Large icons.
Select Administrative Tools.
Double-click on Internet Information Services (IIS) Manager.
Click on View Application Pools
Right-click on the DefaultAppPool. If the status is stopped, start it. If it is running, restart it to fix the error. 
If error persists, go to the following steps.
Click on Advance Settings on the right-pane.
Click on Process Model to expand it.
Click on Load User Profile and change True to False.
Click OK and check if error is resolved.
