Are these units connected to an internet provider?
Do you have a stand alone DVR, or is it by the camera provider?
<>If its not saved on an HDD nor on the Cloud, that would seem odd to me.
Maybe you know too much.
Watch your six.
Most DVR systems use loop recording and erase after a set amount of time.
If the video is sent through a home wireless network to the DVR I would check to make sure no updates have cause a connection issue between cameras and DVR. Same for a cloud based system, insure a power outage or something didn’t cause the cameras to lose connection with the router.
Otherwise, check the HDDs are still working properly. Hopefully the DVR has a way to check HDD health. They are pretty reliable, but failures can and do happen.