OK, when you get to where you sign in after you have clicked on My Cloud under OS3 before signing in click on Forgot Password and follow the instructions.
Also make sure you are using the right password MyCloud.com for the web portal. People sometimes/often use different passwords, one for accessing the My Cloud on their local network, and a different one for accessing the MyCloud.com web portal. Wouldn’t be the first time someone mixed up the passwords.