If Jews do not believe in a hell then why would they be punished for sin?

answer: In Judaism, sin is more accurately known as "missing the mark" - we've failed to live up to our potential. Repenting, making amends, asking forgiveness from those we've harmed and then asking G-d to forgive us is seen by some as punishment. Many also believe that those who aren't "righteous" at death are separated from G-d for a short period (a time of reflection and repentance - being separate from G-d is punishment). This is a very short period (12 months max)