I just installed Lion, and now I'm hanging on startup. What went wrong?
What went wrong was your hard drive is failing. The system cannot find a valid bootable system. You can test this by booting to an external drive (which looks like might have to wait for getting home or to the shop)

If the failure is happening fast - you just had bad luck and it would have failed within days no matter what. Get your stuff off and be glad you got things out before the "house burns to the ground".

If you had a slow failure, then the upgrade could have made things worse. This slow failure can often be made better by zeroing out the whole drive. Bad blocks get marked and for a while, it is reliable. You might buy some months or a year, but it's starting to fail.



Did the technician let you know if she or he felt you had a crisis or a slow issue with the HD?

As an analogy, Imagine a large "house" made of four decks of cards and lots of levels. If the far left hand side is shakey but you only are adding cards to the right, it might stand for a long time. As soon as you touch the left, down it comes.

A system upgrade touches everything - and re-writes the core software needed to boot. Your mac just wasn't up to having critical files written reliably and the upgrade brought those errors to stark light.



Put another way, Lion is running on millions of macs. 

Sadly for you, something about your mac was different than most and the results quite painful to your productivity. The good news is many are going through the same pain - discovering how fragile HDD/SSD storage can be when a critical file fails to get written correctly.

I hope you get everything fixed, lose no data and can start enjoying Lion.