Why is Time machine backup disc unavailable

I did a successful time machine backup this morning less than 12 hours ago. When I restarted my iMac (purchased 2012) it rebooted and gave me that ominous message that the external USB disc was no longer available. I reformatted it after backup to High Sierra about 6 weeks ago and until today haven't had any issues. I can enter Time Machine and see all the backups but 'restore' is faded out. What is going on here!!!