No one can help on that little information. There are probably a dozen or more possible reasons email isn't working.
Has it ever worked?
How did you set it up?
Did you follow the instructions for iPhones on your email provider's web site?
Who is your email provider (DO NOT post your email address in this public forum)?
You might want to review this Help article from Apple: http://support.apple.com/kb/TS3899