the flat wire that connects the top screen to your motherboard has gotten loose or is broken. It would be much cheaper for you to either buy a new top screen at ebay and replace it yourself or look inside and find out if the flatwire is only loose which you can just snap into place before you waste 50/75 bucks on Nintendo's service.
i've had this problem many a time, but mine just goes white, once it went pink. it COULD be a problem with your gamecard, like mine. the chip inside the casing for the game couldbe loose, and if u tilt your ds back, the chip will slide, thus disconnecting ti from your ds, and causing it to freeze. ALTHOUGH, i COULD be wrong. its all random speculation.