I can't access the Sandbox account. My email address and password don't work.
It's possible that you are not yet listed as a member of the Sandbox account.
Creating a Sandbox only copies the owner and all existing administrators' credentials. If you were added or upgraded to an administrator after the Sandbox was created, your profile will not be present on the Sandbox account. In this case, the owner or any administrator who has access to the Sandbox will have to manually add you as a member of the Sandbox account.
Ask the owner or an admin of your account to open the Sandbox link and follow the steps in Adding agents and administrators.
As a newly added user, you will receive the welcome email which will allow you to accept the invitation and create the account. Just like below:
For more information, check Testing changes in your sandbox (Enterprise).