This is why you dont buy pre-built PC. Call up dell of support most likely it will still have warranty. Please explain your question properly about it not starting up.
I believe that your computer might have over heat. I suggest removing the cover to wash out the venting or filter. Once you do that, it might work. Try letting it cool down for a day. Or buying a new motherboard/processor. It might be because they have a circuit breakthrough. You can try changing the power supply. Just saying the computer wouldn't start up is not really enough information..