@Paweł , Just checking if you had got a chance to see the previous response.
Adding to TP, there could be multiple conditions that can cause this error to be presented:
1. The site has reached a billing limit and your site has been disabled.
2. The Website has been stopped in the portal.
3. The Azure Website has reached a resource quota limit that applies to either Free or Shared scale modes.
You may review the error details from developer’s tool /or F12 . Once we observe the traces on the browser window, to fetch more details about the error.
In case, F12 trace show - “HTTP 403 : IP forbidden“, which basically states that the client IP browsing the App is not allowed to access that App service i.e. “IP restrictions” have been explicitly configured on the App service. You can check the list of blocked IP addresses from the portal .
There could be another possibility where the IP Restriction could have been configured using web.config.
Please checkout the suggestions outlined by my colleagues in these blogs/articles for more info:
Azure Web Apps: “Error 403 - This web app is stopped..”
Azure Web Apps: “Error 403 - This web app is stopped..”
Kindly let us know how it goes, we'll follow-up with you further.