No post after storage

Hello,

I've run into a small problem that's a little annoying. I left my computer in my parents basements as I've been in the US for 7 months. Pretty constant atmosphere.
Coming home I obviously expected it to work as I left it, but that didn't happen. 

I got no post. Only a Q LED for CPU. Tried reseating it. No bend pins. I've checked some of the PSU 12V with a multimeter.

Here are the specs:

i5 4670k (was overclocked at 4400mhz)
Maximus VI Formula
G Skill F3-2133C9D-16GTX
Asus GTX 770 
Corsair H100i
Cosair HX750 PSU
Evo 750 SSD

There is no visible damage. I have a sixth sense that the CMOS battery may be dead. Can anyone offer any suggestions?

Kind Regards,

Martin