Video Loss on all cameras to a novice can be daunting to determine the issue. The place to start is with the power supply. Odds are you lost power to cameras. I can’t tell you how many times it turns out that someone unplugged the power supply because they did not know what it was for and needed an outlet to plug something else in. Once supply is connected test it with a multimeter. Make sure you are getting 12VDC or a little higher per channel. At this point start checking connections and voltage at cameras. You should find the issue.