I assume it is mighty model? About 25cm square?
It may be the harddrive is faulty. Check diagnostic option in settings.
Optus will arrange collection and send out a free replacement if it is busted.
Odd that the only issue is it won't record, everything else is fine?
Assuming you've tried different channels and programs?
A soft reset may also help:
To perform a soft reset on you Fetch box: