Webpage shows Squirrelmail login

Hi guys,
I have one webpage for example mydomain.com which worked well, but after few days this webpage stopped working and it is redirected to Squirremail login webpage. Server has static public IP. All other webpages working fine. Settings for webpages which works and which shown squirrelmail login webpage look same. Do you have any idea, what can I check?

Thank you

 

Nevermind guys. I did rollback from previous snapshot and working well. But I cant find any difference in settings, so if you have any idea, what was wrong. Let me know pls.

 

It happened to me twice:
1- It was what Till suggested above.
2- The site client exceeded the bandwidth limit. Once the limit corrected it works again, although I was expecting a page with an error instead of the squirrel login. Forgot to ask at the time.

 

Thank you, guys!

 

You are welcome, but could you please post a hint on the root cause of your problem and the remedy?

 

I am not exactly sure what what was wrong. I did rollback from my few days before snapshot and working fine. I only know, that there was no site client exceeded the bandwidth limit. And true is, that I changed squirellmail.conf while installation process this new server, but nothing more, that is in manual for installation ISPCOnf 3.1b2 on Debian 8 on those sites. Working fine about 2 weeks, after that I did some changes in Server bookmark in ISPConf web app, but there was a lot of changes I am not sure what can cause it. Sorry.