Sometimes, in Mac OSX, the emulator window won't even show up. And the debugger won't be able to connect to it. Also, it won't show up in the devices list in adb. Finder also has the force quit defaulted for the application icon when you try to close the application.
I have the Intel HAMX drivers installed.
The console log shows:
kernel: haxm_error: HAX: vmxon failed (7337f000). Is that another VMM running?
Hopefully, this helps someone, as it was a pain to figure this out.
I figured out that it was the act of me having a running Virtual Machine running in Virtual Box. After turning off the Virtual Box Virtual machine, then retrying to start up the android emulator it started working as normal.