I get sound but no image on my conia m550 and sonwa sdvd300, what do I do?

I was given a conia m550 around six months ago and gave it to my mum who used it less than ten times before it stopped showing a picture (but the sound still works). I figured it was just a dodgy brand and hence dodgy player, but recently my girlfriends parents dvd player - a sonwa sdvd 300 (which funnily enough has the same buttons on the front and EXACT same remote [except for the brand name]) - also has just stopped displaying images to the screen but still has sound (they used theirs to watch hundred's of dvd's both bought and rented)...and it got me thinking maybe this is a common problem which has a simple enough hardware fix.

Any assistance would be greatly appreciated. Cheers.