If you are hosted with the Access Group, please raise a new case online and reference the title of this article.
If you are on premise, follow the steps below.
Recycle the application pool
Navigate to the website by expanding the sites menu in the connections window.
Right click on the site experiencing the issue, click Manage Applications then Browse.
If you are still experiencing a 503 error, then please navigate back to the application pool and check if it is still running.
If it has stopped, navigate back to the Application pool and click on Advanced Settings from the Actions Menu on the right.
Click on Identity under the Process Model sub section.
Confirm the security setting in this window and click OK.
Repeat steps 1 - 3 to confirm if this resolves the issue.