Assuming you were not unlucky and got a second bad box, the most likely cases are
- You left your TV on the same channel more than the auto turnoff setting (i believe it defaults to 4 hours)
- flaky power. Consider at least a surge protector or a UPS
- Your box lost contact with the network, Check the connections
- Bad ONT
- Box just received a download of new software (should be obvious if you were watching)
- Some new bug in software. Don't know of a current such bug, but there have been such in the past.