Firstly, are you using webmail or an email client? (Webmail is where you use the same web browser, such as Internet Explorer or Firefox, as you do to view pages here on AB. An email client is a totally separate program, such as Outlook Express, Windows Live Mail or Thunderbird, that is used specifically for accessing your email account and not for viewing the web).
Then:
(a) if it's webmail, please tell us the provider (e.g.Yahoo or Gmail) ; or
(b) if it's an email client, please tell us which one you're using.
Then there's a chance we might actually know what you're on about!
However, as a bit of a random guess, it's possible that you should EITHER be looking for a scroll bar at the side of the email (as there is at the side of this web page), enabling you to move up or down OR moving the obstruction (that's hiding the lower part of the email) down by placing your mouse cursor over the top of it and then dragging it downwards.