Really it's impossible to say what the problem is. There are sooooo many things that can cause a laptop to freeze, from mechanical like a dirty/non-working fan, to badly installed software, to out-of-date drivers, to a virus or malware. The only thing to do is a hands-on approach by someone familiar with laptops.
If your computer guru can't fix it, and he had it to work on, it's unlikely you'll be able to do much yourself relying on internet advice unless you're better at it than he is. :)
Edited spelling mistake.