How do I fix 'Server not available' error for Find My on iMac?

Have been trying to track my husband's iPhone while he's on his way home from work. Suddenly yesterday, I opened iCloud on my iMac and clicked on the Find My icon and got a flashing glimpse of the FindMy screen which was quickly switched to this screen which it has been giving me now for 2 days. All other iCloud functions normal! I have NOT changed any network settings or updated software...

[Re-Titled by Moderator]