Solution: HTTP Error 503 error may show up for several reasons. So the first step to solving this issue to identify the root cause of this problem. Below are some of the probable causes and their respective solutions for this error: 1. App Pool Identity: If the account with which App Pool is running is recently reset and if the same has not yet been updated in IIS, then this may result in Service Unavailable error while loading the website. 2. The second cause is also related to the account used in App Pool. If you have updated the correct username and password in IIS and if still, you are getting this error. Then you can try to change the account to AppPool Identity from the currently configured account. But please remember, you will need to update this on each website which is linked to that app pool. 3. There is one scenario that may result in a 503 HTTP error (personal experience). If by any chance due to some other organizational change e.g. SCCM Deployment, someone has renamed the MachineKeys (Path : C:\\ProgramData\\Microsoft\\Crypto\\RSA\\MachineKeys) folder, then you may run into the same problem if your website is hosted on the same server. It can be sorted by just restoring the old MachineKeys folder and resetting the IIS once.