Try to rule out the causes... Go to your inlog page of your mail account on your pc and try with your password. If you can access the mailbox, then you know it's your iPhone. If you can't, it's you.
If you can access your mailbox on the web, then make sure you've configured your mail-account correctly. Here is a tutorial how to set-up a POP-account on your iPhone. Here you can find one for an IMAP-account.
And you can always try Google to search for a setup for your e-mail client. For example "How to configure hotmail with iPhone", or so.
And as a last resort, you can delete your mailbox and re-add it. Your iPhone will automatically rebuild your inbox, so nothing will be lost.
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If you're using a POP-account (like hotmail), I could recommend an IMAP-account (like GMail).