I am unable to receive incoming messages from my email account? What should be done

More information would be helpful.
Are these simple messages or attachments ?
Are you using a client, such as outlook express, or webmail.
Is this on one account or more ?
Your operating system??

Check with your email service provider what your pop settings are, Is authentication needed, Do they use secure ports?
Who is your service provider? Gmail / Yahoo / Private ISP?