Rebooting in fail safe mode failure

Hello,

i had an issue with a PC after trying to change the memory on an Windows Vista PC..however the PC was not booting up so i put back the original RAM and when i booted i got the Blue screen of death error. after booting into safe mode and running diagnostics on the PC i rebooted again but still same error. i then when into the BIOS and rebooted in failsafe mode. this then caused the PC to not boot up at all, and the HDD light is not blinking, please help. what cud be the cause and how shud i fix this.\\thanks in advance

Whatever you did in bios, go back and undo !
The BSOD shows a stop code. What is it ?
What caused you to change RAM in 1st place -
I won't insult you by asking if it is the correct RAM and you took
precautions to install it properly.

You can get into safe mode simply by tapping F8 key during startup, immediately after the 1st screen pops up without going into bios.