I do believe in a God or Supreme being and the name of that Deity is Visnu. Getting to your question, what will happen if you do not believe in god? The answer would be based on the laws of Karma you would take a rebirth into a different situation based on previous live activities. Since the spirit is eternal some of us will have to take millions of births until we make it back to the spiritual world or back home to Godhead.
I will not claim to know for a fact, but this is what I believe. If you are a good person then you will have a better situation next life. So there is no need for you to have any worries. That is what I like about the religion/philosophy. Good is always rewarded.
According to the philosophy God created material nature so that the spirits could come and live material lives. Once we remeber God we will get to go back to the spiritual world, but God does allow us to forget that God exist.
There are differnt wonderful realms fo those sould who are good, caring, kind, compassionate yet they are atheist. However, they are temporary worlds.
I hope this helps. You may like to read the Bhagavad-Gita written by His Devine Grace A.C. Bhaktivedanta Swami Prabhupada, if you have not read the book already.