Are you trying to open the Dashboard or do you want to sign in to files.mycloud.com? Are you able to provide an image of the sign in box? What device are you on and is it a Windows, Apple etc. and what operating system?
What does that mean? Do you have a single bay/single drive My Cloud or a My Cloud Home? They are different devices. If you have the My Cloud Home see the dedicated forums for the My Cloud Home: