Open SharePoint Management Shell on one of your SharePoint servers with elevated rights.
Try to get the site collection having trouble with Get-SPSite. See the following command and modify url to your environment:
$s = Get-SPSite -Identity http://URL
(see Microsoft Docs Get-SPSite for more info if needed)
If your second command returns the URL of your site the site exits in SharePoint config.
Then try to delete the site using this command.
Remove-SPSite -Identity 'http://URL'
(see Microsoft Docs Remove-SPSite for more info if needed)
Now as you got rid of the site - or if the site never existed as you have checked in step 1 - you can create a new site. You can do that with this command:
New-SPSite http://URL -OwnerAlias "DOMAIN\JDoe"
(see Microsoft Docs New-SPSite for more info if needed)
Then your site should be ready for use. If the error persits go and download ULS Log Viewer and check your SharePoint logs for the url you are trying to enter. Within this log you will get further information about missing site templates or any other possible error. Please post the SharePoint version in use as well (2010/2013/2016)