LaCie disk working with the Time Machine, as a backup disk

Hello!

I set up my LaCie disk to work with the Time Machine, as a backup disk. I set up a password to lock my disk, but unfortunately I forgot the password I entered in the setup process and now I CANNOT ACCESS MY DISK ANYMORE!! My Mac Book Pro is asking me "Enter the password to unlock the disk", but I don't remember it wright and so, I can't access neither my data, nor the installation program that is by default onto the disk in order - at least - to reinstall the disk from the beginning as a Time Machine backup disk and to set up a new access password. Please, help me to access my disk!

I have a MacBook Pro with retina 15" display, from 2015 running OS X Yosemite 10.10.4. The LaCie disk I'm using with Time Machine as a backup disk is a 3TB 2 x thunderbolt 2 disk.

Thank you for your kindly help!