I've had this problem twice .On the first occasion I took out the battery , then held down the power button for 30 seconds ., then replaced the battery and when I pressed the power button it started fine ,
The second time this didn't work so I unscrewed the panel covering the RAM Memory . I had 2 inserts there , one of which had come loose from its clips . I re fitted it into the slot, screwed the cover back on and restarted it succesfully .I'm no expert and it was an easy thing to do - and it worked ! Apparently the RAM insets can come loose after a while. I saved myself a hefty bill too.