Dell Inspiron laptop will not start up

Hello, recently I have just switched the hard disk of one of my inspiron laptop to another laptop, also an inspiron. However, I have encounter some problem.

"When you press the power button, If your laptop powers on, the lcd panel is pitch black blank screen, not even back light power, no dell startup screen, you hear the fan spin up, you hear the hard disk spin up, in this state one of the 3 LED lights blinks for 7 seconds (I think numeric keypad), and then if everything switches off silent. The only LED on after that would perhaps be the battery charging indicator. it could be a memory DIMM gone bad, If you have two dimms try one, then the other to confirm"

That is the description of the problem. When I go online, they suggest that my memory DIMM might be spoilt. May I ask, How then can I solve this problem? Is it a hardware problem whereby parts of the laptop needed to be changed or?

Thank you for your reply.