Array becomes degraded when member drive becomes missing; in your case, it can be that one member drive becomes out of sync or some other reasons.
To fix it, you need to rebuild that array. Carefully mark the drive in degraded array and completely take it out (swap out). Now insert the drive that becomes missing/corrupted/out-of-sync and (prob. show you as not-in-use or available) delete that drive so that it becomes available (data will be deleted). Then insert back in marked drive and rebuild the array using available drive. It should now make RAID-1 array again.