Having used Onkyos in the past (he said, to establish his credentials, as seems to be required now) I'd wonder whether the receiver is responding to the controls on its front panel?
If no, could be that the microprocessor has locked up, in which case, set the receiver to standby, unplug the mains cable from the wall, wait 5-10sec, plug back in and switch on again.
If yes, could be that the receiver has defaulted to a different remote code to that set on the remote handset. Might be worth trying different remote code settings on the handset. From the manual:
Remote ID
When several Onkyo components are used in the same room, their remote ID codes may overlap. To differentiate the AV receiver from other components, you can change its remote ID from “1”, to “2” or “3”.
Note
• If you do change the AV receiver’s remote ID, be sure to change the remote controller to the same ID (see below), otherwise, you won’t be able to control it with the remote controller.
Changing the remote controller’s ID
1. While holding down RECEIVER, press and hold down Q SETUP until the remote indicator lights (about 3 seconds).
2. Use the number buttons to enter ID 1, 2, or 3. The remote indicator flashes twice.
You can always cycle through them all and then come back to one. Or you could just check on the receiver itself which remote setting it's using ( see the manual p48 ).
Beyond that, I'm at a bit of a loss.