Happens to everyone at some time or other, and it can be anything.
Next time, try selecting normal with a boot log. That way you can read the text log afterwards (usually directly under C
) and it may point to an errant driver.
Common causes though are poorly seated RAM, graphics drivers, or just in need of a good defrag.
Hope that helps a bit.
------------------
If you're not on the edge, then you're taking up too much space.