Welcome to JustAnswer,
I am Pete, and I'll be happy to answer your question.
When the recorded programmes no longer appear, but the remaining disk space still shows that a percentage has been used, this is likely due to the file structure of the internal hard disk becoming corrupt.
It is not easy to attempt the recovery any recorded programmes.
The hard disk would need to be physically removed from the Sagecom box and connected to a computer using a hard disk caddy.The format of the drive is non-standard, so the computer will not be able to see it unless you use some special software, for example DVBTX or a similar software program.
To recover your Sagecom box and make it workable again, the hard disk will need to be reformatted to fix the damaged file structure. If the box is quite old, there is a chance that the hard disk could fail again, and it may be advisable to replace it at this stage, with an equivalent or larger sized disk.