"The last backup could not be completed." Last Backup: 5/12/15 at 11:51pm What should I do about it?

My i-phone 4s has recently stopped backing up to the i-cloud

When I click "backup now" while connected to a known reliable wifi, It says Backing up / estimating time remaining for about 60 seconds then says, "The last backup could not be completed." Last Backup: 5/12/15 at 11:51pm

What should I do about it?