Sounds like you have an intermittent connection soemwhere within your monitor. Give the video in cable and its connector a thorough (but gentle)"wiggle". Try turning the monitor gently upside down just in case there is something loose inside that may dislodge. Switch on again and hope for the best. If that doesn't work, I fear you may be down to fiddling inside the monitor (if you feel up to it)or replacing it!