Big Problem pls help

[BaronKiller]

Not sure where i shld post this problem but anyways...

My computer couldnt start up..i pressed the power button and the fans goes on and u can here the beep but then the power juz suddenly  goes off.then i tried unplug the 6pin connector of the gpu and the computer became fine .so i tried using a gpu that doesnt need a power connector and the computer works juz fine...is it becuz of overloading??but two weeks ago my oringinal setup works juz fine..why it suddenly happened

My system is

I5 2400

Gigabyte h61m p25 b3

Asus r7770

Toshiba 7200rpm hard drive

Magix 400w psu

[Computer Encrypter]

Is this the first time you tried to boot your computer?

[GDRRiley]

get a better PSU 

[BaronKiller]

13 minutes ago, Computer Encrypter said:

Is this the first time you tried to boot your computer?

No ..it works fine totally until one day the problem start happening

[BaronKiller]

7 minutes ago, GDRRiley said:

get a better PSU 

Will 500w be enough? So is it becuz of overloading of the 6 pin connector..but it worked fine earlier on

[Computer Encrypter]

The psw should be fine but the gpu might have died or something like that

[BaronKiller]

11 minutes ago, Computer Encrypter said:

The psw should be fine but the gpu might have died or something like that

I mean even if the gpu died the computer should still start up right?

[Computer Encrypter]

20 hours ago, BaronKiller said:

I mean even if the gpu died the computer should still start up right?

So the computer didn't boot an there was no whirring of the fan?

[Danscu]

Your power supply may not be powerful enough to support your system. Also there may be something wrong with the GPU, which could be causing the problem. But I'm just guessing