Ok i'm new here and i have a problem. The ventilator at the back one day just stopped spinning and i dont know what to do if i tried to play games the computer shutdowns and restarts (sometimes) so any tips how to make it work again ?

Case fans can die
The usual occurance is internal dust, therefore removing all internal dust can possibly help
In most cases it's a matter of replacing the fan

There is also another fan, close to where the power cord plugs in
This is actually the PowerSupply fan. Once this one dies, users tend to purchase another PowerSupply (as PowerSupplies are sealed units, and not meant to be opened)

You will need to work out if it's the case fan, or the PowerSupply fan, and then have it replaced

From the symptoms, it sounds like the power supply fan. A case fan, especially in an OEM computer doesn't cause computer shutdowns...

Have someone replace the power supply

Ventilator fans (also known as muffin fans) - are extremely cheap - just a couple bucks. I used to sell them when I ran a salvage business about 20 years ago. Don't run your computer without adequate cooling. You can seriously destroy the rest of your components. Spend the $2 to $3 and replace it. If you have a failing PSU - (the only way to test is with proper tools), replace that too. Fans fail for a variety of reasons - dust, bearings, and bad voltage.