Isn't it obvious? it's Broken, dispose of it responsibly and get a new one.
You need a new PC. Don't turn it on again.
It's died. It needs a new one. You may be able to remove the hard disk which contains all your data and programs.
perhaps the power supply has burned out
get a new one
Sounds like the power supply blew. Happens all the time. Easily replaceable.
Trash the junker
It needs to go to rehab.
Find a local computer repair specialist
(Not the service counter at a big-box store)
and have it looked over.
Sounds like a power supply failure. Unfortunately, a power supply failure is often catastrophic and will destroy other components in the process.
Of course, do not send any more power through the pc or the components. Inspect each component where the power supply connects to it for damage (usually melting/burn marks) on or around the connectors. If you notice melting or burn marks, its very likely that component is dead and will likely need to replace. Also, try using your sense of smell to determine whether or not the components may have been affected. If the components smell burnt/fried, especially after taking them out and letting them air out a bit, it also may mean they are damaged and may need replacement.
If components show no signs of external damage, you can attempt to connect them to another pc (or your replacement pc) to see if they are still functional. This also includes USB devices that were connected at the time. They may not show signs of damage but they still may be. Use another pc to test them.
This is what happens when you don't use a decent quality power supply, and/or when you let too much dust and dirt build up inside the power supply and pc. Often times on pre-built computers, the power supply is very low quality. So its recommended you replace the stock power supply in many cases, especially if you do any hardware additions or upgrades and/or overclocking.
When you buy decent quality power supplies, sometimes they will replace the damaged hardware if you send them proof and they find the cause was a defect in the power supply. (the really cheap brands won't do that. probably because they know their shitty psus cant wait to catch on fire)