The DVR could have accidentally put in SD mode. Hold the EXIT key and see if it will give a message about whether it is HD or SD mode.
If you have a VCR or DVD recorder connected to the DVR. That will cause a warning about not being a HD cable and asking to change to SD by holding down the EXIT. I use it all the time with my DVD recorder. Some people have done it by accident.
Depending on your TV the difference can be very noticeable.