External monitor crashes Macbook

I’ve been using an external monitor (Dell U2413) with my Mackbook Pro (OS 10.10) for the past four months without any problem. Now when I plug in the external monitor the Macbook’s screen goes black. It’s very dim but I can see and move the cursor. Other than that it appears that the machine has crashed. I have to unplug the monitor and do a hard restart to get the Macbook’s screen back.

I’ve tried booting the machine up in safe mode. The first two times I did I was able to plug in the external monitor and use it. Now when I boot into safe mode and plug in the external monitor it stays black. I get a warning saying that it’s entering power save mode. Though it’s black, Displays in Systems Preferences, shows that I have an external monitor attached.

Any ideas what’s wrong or what I can do?