Seagate Backup Plus 5TB is connected but iMac no longer recognizes it

My iMac has always backed up the Time Machine to my Seagate back up plus 5T, but recently it does not recognize the Seagate and indicates it is not connected, even though it is connected.

[Re-Titled by Moderator]

Original Title: seagate backup plit 5t is connected but imac no longer recognises it