Outlook 2003, SP3

After using Outlook for a long time, all of a sudden I am not able to
send or receive emails. When the program is open a box continuously
pops up called Enter Network Password. The server, user name, and
password lines are already completed. It does not matter if I hit OK,
Cancel, or check the box for "Save this password in your password
list." Nothing changes and the box opens again.

What can I do to fix this?

Thank you.

Caryl