When you join IMDbPro, you'll be prompted to select a password. Unless you've changed the password since you joined, you can log into IMDbPro using your registered email and password.
The only case when you might not have been prompted to choose a password while joining, is if you were already registered with IMDb.com. In this case, your existing IMDb.com account information (email address and password) was used by default. This lets you access both sites without the need for two separate accounts.
If you were not prompted to choose a password or don't remember the password for your membership, you can get a new one. To reset your password:
As soon as you've created a new password, it's active.