[SOLVED] Blog suddenly won't load with any version of atahualpa theme !

I don't exactly know how it happened but I have been having a few bugs lately with my website. Eventually I am having now a White Screen of Death when trying to access to my website but somehow this only occurs only with the atahualpa theme (whatever the version), even though I was using it without any problem until then.
I have tried :
- reinstalling a fresh copy of the atahualpa theme --> won't work
- rebooting all my plugins --> won't work
- Changing the CSS from inline to external file (whatever it means) --> works fine but my category menu is all messed up (it lost most of its styling (colors, padding...)

I recently upgraded my wordpress version to 3.0.4 and my wp version to 3.6 (now switched back to 3.5.3) and wonder if that may have caused the issue.

Please help me ! I am brand new with webmastering and am getting completely lost with my options and am a bit scared to mess things even more doing a bad move.

Thank you all for your help !