SharePoint site inaccessible in SharePoint Server
APPLIES TO: 
2013 2016 2019 Subscription Edition SharePoint in Microsoft 365
There are many different root causes that can lead to a user or admin encountering the site inaccessible error in either SharePoint Server or SharePoint in Microsoft 365. The root cause can be anywhere between the browser and the back end database or in one of the supporting systems, like DNS or the network. When you encounter the site inaccessible error, or other slow or no response SharePoint Server or SharePoint in Microsoft 365 sites you can start your troubleshooting with this workflow.
The site inaccessible troubleshooting workflow
The workflow is only available in Visio format. You will need either:
Visio 2016 or Visio 2013
Visio viewer, available here
Follow these procedures to use the site inaccessible troubleshooting workflow.
Using the site inaccessible troubleshooting
Download the SharePoint Site Inaccessible Troubleshooting workflow and save it locally.
Open the workflow in presentation mode.
Add an indented paragraph under a numbered step by placing cursor before the /para tag, then press Enter.
Locate the appropriate starting place in the workflow and begin. Note that a list of multiple links will appear when you click on any of the boxes that say click here for help.