Mail has stopped working WHY ??

Help...

This is my first ever Apple product, just treated myself to a macbook pro .... set up the mail account and for about a week it worked fine with no problem, then all of a sudden NOTHING !! mail does not collect any more... I didnt change any setting or touch anything, whats happened ?? My mail account is with BTYahoo, and I have tried to read other questions posted, but they are all rather out of date..

How do I fix this?

[ to be honest I have a million questions, and I find it difficult to get any help on using a mac :-( ]

I would be grateful for a fix for my Mail prob

Thanks