Unplug the box from the mains, wait a moment and restart. Helps with some issues.
How do you know the remote is OK? have you checked (use a mobile phone camera) that the IR LED at the business end is actually lighting up.
Are the remote batteries OK?
Has some idiot put an ornament in front of the IR sensor on the box (Yes, it HAS been known)
This may help:
Alternatively you could shell out for a replacement remote and hope that was the problem. EBay is useful