give details....
What happens when u put the umd in, does it spin? does it make a clicking noise? does it make this faint tick noise? when you press down the black pressure switch does the laser turn on?(use peripheral vision)
it could be the power ribbon cable not being connected, the main ribbon could be partway out, the mobo could be screwed in too tight or loose, the lens could be dirty, something small could touch/interfere with the gears...





