Won't turn on. (by 2 users)

Hello, I was working on my machine, when it just shut down, I could not move my mouse, and just a box was on my monitor saying something about power being shut down, so I turned it off, and now it will not turn back on. I took off the back and checked and cleaned it out, wow, what a mess, anyway, I hooked it all back and still nothing, I have been having trouble with the big fan coming on a lot before this happened, when I ran my card games, or whenever a program would start up. It had happened earlier in the day with my granddaughter, and I just shut it off and it came right back on, but that night, I was playing a mean game of pinucile when it was just gone. Can you tell me what happened? Please.