Sounds like the MBL is unable to boot properly because of a problem with the OS. You should first try hte reset button on the rear of the device (look in manaual for how to do it exactly). If that doesn’t work, then you have to do a debrick using one of the guides on this forum. You will not lose your data.
take the disk out of the drive and try to mount it to a linux-running PC. You can use a Liveboot linux CD or USB stick. From Linux, you should be able to see all 4 partitiions and hopefully see the data in them. The 4th is your data partition. the other 3 are for the OS. If you can’t get any of that to happen, then you might have a dead disk - so send it back for a new one.