All computers freeze or crash at some point, even macs, the difference is that Macs can lose an App and carry on and they don't crash anywhere near as often as your standard Windoze PC.
It could have been a number of things, have you repaired your permissions etc. lately?
it's no biggie unless it happens a lot, in which case you may have hardware problems.
My 17" PowerBook was a complete lemon when it first arrived, you couldn't move it without it crashing, Apple replaced the machine and this one is still running strong nearly 2 years later.
Yes, it has frozen occasionally, but only when I've been doing silly things like rendering video and answering email at the same time.