Could be a number of reasons for this, such as faulty components (capacitors, semiconductors et cetera).
Some Macintosh computers can even show no-video symptoms if the internal (logic board) battery is weak/bad.
Before you continue, it might be a good idea to test a PRAM reset (link below), and to try starting from a disk tools floppy.
Finally, a warning. Please note that opening a compact Mac can be very dangerous because of the high voltages involved, and must only be carried out by someone with the necessary expertise.