There is not much information to work with. Please provide more details:
Has it ever started since it was rebuilt or is this the 1st start that is unsuccessful?
What troubleshooting have you already done?
Can you tell if it is fuel or spark related that it won't fire?
Was it running before you rebuilt the motor?
That information would be a good start. I am not sure that I will be able to help in your particular situation, but I know for anyone to be able to, more information will be needed.