Site not working (503) without typing https://

Hi guys,

I can’t figure out why the website of the company I work isn’t working when I go to https://www.mysite.nl/page. It gives a “503 Service Unavailable error: The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.”
It only works when I put https in front, like https://www.mysite.nl/page. Which is really annoying since potential customers would just go to www(or even without www).mysite.nl/page. Also our ads + radio campaigns are all set like this, not to mention links on partner websites.

Hopefully there is someone who can help me and know what to do :).

Thanks!