Assuming you haven’t changed the code on the remote because you have multiple Humaxes in the same room…
If the buttons on the box work and the box is on, power down using the right hand button. Wait for the disk to go silent. If the box is already off you should be okay. If the box won’t respond to the off button on the box you’d have to follow the next step anyway! Switch off at the plug, wait a few seconds and the switch on. When the box restarts you should be able to control it with the remote.
If you have multiple Humaxes in the same room, or have inadvertently changed the remote code see https://myhumax.org/forum/topic/multi-code-function-hdhdr-remote-2