Please be more specific. Please include exactly which model of Mac you're using, which version of OS X you're using, and what email client you're using. Also please describe the exact procedure you're using to send email and any and all error messages that appear along the way.
As it stands now with the information you've given us, though, the problem could be anything from a catastrophic electrical outage all the way down to you simply not pressing the "send" button. Please elaborate greatly and we'll do our best to narrow down the problem.