HORNETS7

I have a an tone 4 ch dvr which was working fine until a few weeks ago ! only a small problem but its recording 4 channels as normal but if / when i try to watch a recording back all i get is a blue screen! so i cannot watch any playback? all the time scales show that there are recordings and it is recording and i do get picture outputs when whatching the cameras live but just cannot view playback footage? i have tried a part of the menu which says system restore but that didnt do anything! any ideas? thanks in advance

hi thanks for replies ! i bought the unit new and worked fine! i have previously watched playback recordings but it won't now? ive just tried a format of the hdd but it still wont playback! it cant be the out feed as the cameras live feed uses th same plug if i unplug it the cameras go off! if while in playback the screen is just blue and the time lapse shows as if its playing but no pic if i unplug the video out the screen goes black and plug it back in the screen comes back playback blue!? anymore ideas