Check your error logs(computer/manage/), see if anything presents itself; Leaving the computer on won't cause to much issue unless your running up the resources of your computer. Heat wise, put your hand on the tower, if it feels like 80c, then it is hot to the touch, removing the side panels will cool it signifigantly, if you have pets in the house though don't;
Ultimatly you would want to know why your computer is freezing,
how often does that happen, is it in relation to any programs you run or games, when did it start to freeze.