MacBook Pro won't boot past blue screen with spinning wheel

[space-time]

friend of ours has a 15" MBP (not the Thunderbot model, previous generation, I believe 2010 model) and sometimes when she turns the MBP on, it will get to a blue screen with the spinning circle and nothing happens after that.

She stopped by yesterday and I did some troubleshooting, I check HD, repaired permission, restarted several times and all seemed fine. The one more time we started and it happened. We let the circle spin for 10 minutes, nothing happened. She said she let it spin for much longer (one hour) and nothing happened.

I pressed the power button for 5 seconds and then I started again, MBP stared just fine this time. no error messages. I checked the HD again, repaired permission, all seemed fine.

What is this and how do I fix it?

[decay]

safe boot? (hold shift)

[space-time]

decay wrote:
safe boot? (hold shift)

did that after I saw that problem happening, and then rebooted in normal mode, and now I am curious to see is she has this problem again

[decay]

could also try downloading & installing the Apple Combo update for whatever version of OS X she's running.

[Speedy]

Diskwarrior.