Is it rebooting when it freezes? I had this problem about a year ago. It would freeze, then reboot a few seconds later. I firmware update fixed it for me
I contacted Hikvision support and they also recommended a firmware upgrade which I did. Lasted for 3 days and then it started all over again. So last Friday, I replaced it with a new NVR thinking the NVR had the issues but gave the same result. So my conclusion is that it might be a hard disk issue.
BTW, after reboot it would work for maybe 2, 3, 4, maybe 5 hours it depends and they freezes. Even the mouse pointer doesn't work.
Will check the hard disk and replace tomorrow and see how that goes. Am using the WD Purple 4T hard disk for surveillance.
Thanks
Shadi