Clients showing offline in SEPM

I have 1.8Lac machines in our enviornment and today when i checked the SEPM home page it showing 1.57Lacs machines as offline including most of the GUPs.

Any idea how suddenly this haapened? All the SEPM services are up and running, i have restarted them once.

On the client side also it is showing as offline, so no reporting issue it looks like, somehting else. Please provide some solution ASAP, this is critical.