My Passport for Mac 1TB stuck on preparing backup

I’m using my 1TB drive as a backup for my MacBook Pro, and it suddenly refused to stop working almost 6 months ago. It worked for most of the first year of having the laptop (I have a 2018 model, and use the Thunderbolt adapter to connect the drive). I will leave the drive plugged in for hours and it will never get past “preparing backup”. What can I do to fix this?