Based on your description, I can suggest as follow:
Check the web site properties in IIS. Under home directory tab, check the application pool value and then open that application pool from the application pools
(separate node in the IIS tree). Click the identity tab and see who is the pool assigned to. For the sake of testing, you can select "Predefined" and select local service from the drop down. Restart IIS and see if things work out for you. Http 503 error is
generated by the IIS and it happens when web server sends data stream back to the web site.
Verify that all SharePoint services are started, this error might occur because of changing the service account password. You
may need to change the new password to application pool or SharePoint timer service. Click the stopped application pool -> click advanced settings ->Identity ->click the user to retype the user ->Application Pool Identity dialog ->click set
-> manually type the user name and password. Then restart the server.
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