missing e.mails.

[sunnyday2d]

Hi all,
i wonder if any one can help me with a problem
i am having, when i come away from my computer,
i usually leave it on so i can hear when my e.mails
arrive, but the last few days afer a short while the screen
goes dark, when i then click the mouse the blue screen
saying user comes on, and i dont hear my e.mails.
any help appreciated. ( i have tried system restore but
makes no difference. )