What happened to the DVR space?

Last night I had lots of shows and several movies saved on the DVR, with approximately 30% disc space remaining.  This morning as scheduled recording was about to begin, I got the message that there was not enough space to record the scheduled show. When I checked, I found that some 3/4 of all of my recordings had been erased.  I rebooted the DVR and nothing changed.  I erased what I had left except for 1 movie.  With one 4 hour movie recorded on the DVR, my disc space was at over 50% used.  Can you please tell me what has happened to all of the disc space I had on my DVR?