Add Content DB in Site

A few things to consider :

1. How did you reattach the content database? Did you run "stsadm -o addcontentdb -url http//[URL] -databasename WSS_content"? Try that , if not already done.

2. Is the version of the content database you mounted and farm content database the same? If not, please upgrade both servers to the same built number.

3. If nothing of the above works , then try below :

$app = get-spwebapplication –identity “url of the problem web app”
$app.contentdatabases.count

Run SQL query 

$db = get-spwebapplication –identity “url of the webapplication that had the null entry” 
$db.contentdatabses.delete(“guidOfProblemDb”)

If you get null reference exception, then run the following command.

$db = Get-SPDatabase -Identity "guidOfProblemDb"
$db.Delete()

Update site map table in Config DB :

$db = get-spcontentdatabase -Identity [insert guid of cdb here]
$db.RefreshSitesInConfigurationDatabase()