Possible reasons:
1- Server-side error
2- SSL Error
3- Infected Plugins
4- Infected Theme
5- DNS issue
The solution in ascending order:
1- Deactive SSL inf secured is active, if you don't have SSL try to activate it, you can get Free SSL from a different website, Cloudflare is recommended.
2- Deactivate plugins one by one and check if the issue is resolved or not. One deactivation of a plugin if the issue is resolved means the last deactivated plugin has some issue. delete that plugin.
3- Change the Theme and Delete the existing theme and check if the issue is resolved.
4- Check DNS, and make sure that there is no issue connecting hosting with the domain.
5- If above all doesn't work you should change your server. Move your site to new hosting and your issue will be resolved.