maybe a loose connection betwene motherboard and lcd?
to open the device:
1. take battery cover off
2. take battery, sim and micro sd out
#you will need a special screwdriver in the shape of a *
3. take out the 4 screws near the where the battery goes
4. take back cover off, this will unclip
5. another 4 screws to unscrew
6. unclip front cover
#be carefull now
7. there is a ribbon connected to the motherboard from the lcd, unclip it by lifting the small black clip
8. re-connect the ribbon and put the small black clip down to lock it back into position
9. put your device back together
10. if your device does not turn on then its either the button for the power is obscured or if the device plays sound but no image on the lcd then the lcd is not connected properly..