I have a 2010 G37 Sedan and I haven’t been driving it much, about 3 months ago I let it sit for a Month and I had trouble starting it, it was sluggish or it wouldn’t start at all. I started driving it again and it went back to “normal” Well fast forward to now, I let the car sit for a week and A cold front came through Now My car won’t start. If anybody has a direction to point me thank you!

 

Dude, it's your battery. Time for a new one.

 

How old is your battery? Likely the battery needs to be replaced. However,

You may want to consider investing in a battery tender (maintainer). Being that you are not driving your car "daily" and your car sits idle for a week, or longer, at a time, your battery slowly loses some charge. The cold weather only exacerbates that issue.

Judging from online reviews this one from or this other one from seem to be the reliable favorites.