Are these 2 sentences the same?

Have a grammar question. I'm learning "explaining a reason" and am wondering if these two sentences mean the same thing in politeness and if they are both grammatically correct??

寒かったんじゃありませんか。
寒くなかったんですか。

Thanks

The first is stating that you are not cold, the other is asking was is cold or to define "cold".

Yes, they are both grammatically correct. They mean about the same thing, asking "weren't you cold?" "didn't you find it cold?"

Just the difference in nuance is that the first one would be more compassionate (you must have found it cold, correct?), and the second one is a plain question out of wonder/curiosity (you didn't find it cold??) In that sense the first one is a bit more roundabout, a bit more polite.