There seems to be a huge lack of information on this so I wanted to share it here. If all of a sudden your MacBook or MacBook Pro refuses to start and all you get is the rapid flashing sleep light then the problem is most likely related to the RAM. The MacBook tends to have RAM that is tough to install and, in mnay cases, you have to push the RAM into the slots MUCH harder than you would expect. Since this is the case it is also highly likely that the RAM wasn't 100% installed at the factory and simply the normal moving of the laptop can losen it enough to create this issue.
This isn't really a HOW-TO but just a bit of information for if your MacBook suddenly gives you the rapidly flashing light :)