IFindex has changed on hundreds of nodes

i have come into work today and i have several hundred nodes,  where the ifindex has chnage from the normal say 10101  to -1

and so now i have lots of nodes not collecting any stats,  all my maps are now wrong

as the only way i see to fix this is to delete the (-1) index and re add the index back in, and have tested one and it is the same index as it was ie 10101

i'm running  10.5

odly is seems to be most of the nodes i run SNMPv3 on