If you look in your device settings does it say that auto-delete is switched on?
Old recordings will be automatically removed if they aren't touched for a long time, especially if your had drive space is filling up and more space is recommended.
There is no way to recover what is already lost, but you can switch off auto delete to avoid this in future. If, after you switched it off it does this again, then it may be an indication that your set top box has developed a fault.