HP Monitor "Going to Sleep"

Hello everyone!
First of all, I would like to apologize if the topic is not in the right section; for me, the world of computers is almost like Chinese :s

I have a problem with my HP computer or rather its screen! (It's Windows Vista)
It has always worked perfectly until two days ago when suddenly the screen went black while my sister was playing something, and it said "Monitor is going to sleep," and I couldn't get it out of sleep mode.

I've searched online, apparently, some people had similar problems, but I couldn't find exactly the same case, so just to be sure, I prefer to ask here :)

So I have to restart the computer every time!
The startup goes super well; it’s quick and just like usual, but when I reach the desktop, I have about 3 to 6 minutes before it goes to sleep... I really don't understand anything :(

I left it and today I tried again, and this time there are some changes!
It says "No input signal" "monitor is going to sleep," either one of the two messages in succession or one of them... >_<
And sometimes, when I shut down the tower (but not the screen), the message "No input signal" stays...

There you go, I hope it's not too confusing... :s
And I apologize in advance because I might not understand everything at once from your responses... :x
If you need information, don't hesitate! (And even if it’s something obvious for you, I would appreciate it if you could specify how I should check and provide you with the information, thank you :) )
Thanks in advance for your help!!