A Playstation is basically a processing core with a graphics array and some RAM similar to some personal computers, for many reasons these systems freeze and need to be restarted.
First thing you should do is unplug the unit and restart. Some of the PS2 Units start to freeze when they get to hot, it could be the cooling fan has stopped working. Your console could also have a dirty laser that reads the PS2 game disc. You could buy a cleaner.