First start by putting your hand behind the power supply fan out let (its the big outlet at the back with usually circular grids. You should feel a small draft and maybe here the fan.
If the above is OK then turnSounds as though one of your fans has packed it in -
TURN THE COMPUTER OFF AND PULL THE POWER PLUG OUT - then open the case. Stand back and turn the power and computer on DONT TOUCH ANYTHING - see if the fan over the CPU is operating.
Alternatively your computer may have come with heat sensors which can be read from the bios setup - look in your computer manual to see if you have that option - sometimes its not in the bios but if your CPU has heat sensors you should have received a program that monitors your CPU temp and fan speed.