You can request a new password reset email be sent to you via the sign-on page (look for the "forgot your password?" link.
This email will be sent to the registered email address for the StumbleUpon account. For privacy reasons, we will not send password details to any other than this address.
If you continue to encounter problems, please let us know as clearly as possible what you have tried, and what error messages or error conditions you observe.
It would be helpful, too, if you might tell us your StumbleUpon username. Commonly, this sort of problem can indicate that more than one account has been created, and we can use this information to check that angle out.
Just one, general tip: if you have used upper-case characters in your username, email address or (particularly) for your password, please make sure that you accurately repeat those details using the exact same case.