Can anyone please help me out ??

Actually I got an 2gb transcend DDR2-800 Ram, so I opened my CPU and attached that to a slot.
(Previously I had 1.5 GB of ram)
Then I reconnected all switch & started my pc,?it started normally & worked as normal.
But when I restarted my pc it gives a message that " there is no bootable device, and many other text"
Then I switched the plug off and then again switch it on, but now the windows shows some message and there are many option that how you want to start windows like ' Safe Mode ' , 'Start Windows Normally ' etc.
I selected 'Start Windows Normally'
But the windows did not start it just loads-loads & loads nothing happens.
Now I whenever try to start my PC it gives either the options on how to start windows or it gives an boot error.

What is the problem and what is its solution ??