Mail will not send - Snow Leopard

My mail was working fine up to yesterday but to-day would not send mail. Receiving is ok. I checked with the ISP and all settings are right but the smpt continues to stay off line and the send times out.
Something happened overnight!
Advice?

Hi Roger Welcome to the forum. Could be a number of things but you do not include any info re ISP, Hardware, OS or what u did to your Mac yesterday so crystal ball a bit opaque.