If needed, you can reset your Developer Sandbox Account password using the forgot password link: https://sandbox.authorize.net.
You will be asked to provide the answer to the Secret Question which is usually your Pet's Name. If you have not updated your Secret Answer, the default answer is Simon.
For more information about Developer Sandbox Accounts, please check:
Hi, I've tried resetting a password for my Sandbox account, but now I'm being asked to provide a city or town of my first job. "Simon" is not the answer :)
It's interesting that I've had the password securely stored, but it simply stopped working, although API keys do work properly.