My computer wont turn on after i installed a new graphics card

The most probable cause would be that your PSU is not quite up to powering the new more powerful card.
Yes i have just checked and the 250 uses twice the power of the 240.
I'm guessing you have had to rig up a second PCIE cable for the card using a 2x Molex to single PCIE cable adapter ?

There may be different versions around but the 250's i have seen use 2 X 6 pin while the 240 does not use a PCIE power cable.

As i said i have checked it out and there is no way you could expect to run that card on a 300 Watt PSU even a good branded one.

Mactronix  

A 300w PSU is not enough to power a GTS 250. To use your new card you will need to upgrade the power supply.
This would do the job;
http://www.newegg.com/Product/Product.aspx?Item=N82E168...