my toshiba laptop shuts off ?

much like the dell discussed here. When I start it I hear the fan start for about 2 seconds then power goes off. I tried all the suggestions made here but not working. The laptop is only 1 1/2 years old and out of warranty. It started out intermittently shutting off but would run for a little while. Sometimes it would stay on an hour sometimes 1 minute. I took it apart and cleaned all the dust out. Put it back together and will not power up. Please help.