Is the hard drive full?
Your box may have recorded as much as it can and now it finds no space to record any more programmes.
There is normally a disk space indicator somewhere in the menu and this will confirm how much of the disk has been used.
If it is full, go through the list of recordings and delete things you have watched or don't want to watch. (You should do this regularly to avoid the disk filling up.)
Bruce, October 2013