My white MacBook (early 2007) has just lost the ability to start up, at least intermittently. Doesn't get past the initial grey screen with the initial spinning indicator. In Single User mode, never gets through the messages; it freezes and I can't get to a command line. In Firewire Target mode, my other computer can see that the MacBook is there but can't mount its drive.
It has been acting strange for a while, but this is the worst it's ever been. It has now twice, when I did get it to start up and was using it, just frozen - no kernel panic screen, but a spinning cursor and no ability to get control of the machine.
It's clearly a hardware issue at some deep level, but that's all I know. Might be the hard drive, might be USB which has been acting funny for some time and did give some weird messages during one Single-User Mode startup I tried.