Why am i not getting any power to my computer, after I have replaced
the power supply?

Very general question:

First Step: Test your outlet with a circuit tester ($3 at hardware store)
Second Step: Assuming internal power supply (ie not a laptop), did you
make the proper connections and is it powerful enough? I you know your
motherboard then you can find a schematic on the web.
Third step: If that doesn't work, you can try another power supply
(incase you have a dud, that happens alot) or you have a blown
connection on the motherboard itself.

Maybe your switch is faulty or a fuse has blown somewhere?

Hay,

    Don't worry. Just follow these steps.

 1. Just remove your RAM, & again put it back.
 2. Remove Your battry & again Put it back.
 2. Put the cable properly that is comming from SMPS to Motherboard.

See, the problem is solved.

You are probably using a ATX modle of power supply....There are some
small pins on your motherboard. Also there are some short cables with
'power' written on it. All you need to do is search for 'power'written
on the mother board next to the pins I am refering to. Plug it in and
your computer will power on. I am sure this is the problem