Bootcamp mac os ok windows not ok

So here goes.
I have a late 2013 iMac running the latest version of mavericks.
Last Saturday I got around to installing windows 7 with bootcamp and then after everything went well I upgraded to 8.1. Everything worked, installed chrome as browser, rebooted a few times and everything worked ok.
I then went back to OS X by pressing the option button and mavericks booted no problem. Several hours later I noticed my screen was blank when I tried to wake up OS X so switched off and booted again as normal. Happened again last night so did the samething, switch off and reboot. I checked disk utility and everything ok.
I then wanted to go into windows so switched off then switch on whilst holding down the option button.
All I get is a white screen, held the button for 5 mins and nothing. hard boot again back to mac os, no problems, reboot, hold option button and then time had kept something on it and went to watch CNN, came back 10 mins later and same white screen.
So I can boot into OS X ok but cant boot into windows. What am I doing wrong?

I managed to fix this. I think it is something to do with the external drives. Unplug them from your USB/thunderbolt posts and it works.

Also downloaded Boot Champ so can now boot into windows without physically unplugging the drives.