A tank I"m watching for someone is overflowing
 

This is happening right now and I don't know what to do.
It is a reef tank and I don't know what to do.

I know it has been a while btut my first suggestion is to shut it all down and see if you can find anythng that tells oyuhow to reprime the system. It sounds like a suction was lonst on the return hose, if the sump is overflowing or the drain hose if the tnak is overflowing. I wish I would have saw it earlier but I hope you got it under control.

Welcome to the forum although I wish it was under better circumstances.

f4a has the right idea. Either the retutn pump has failed, the return hose has come loose or is plugged, or the siphon from the main tank is plugged.