Why won't my computer boot from any devices?

Question

Windows

The computer is old, there was one memory bar for 1 GB DDR2 800, everything worked, but there was clearly not enough memory for 7, I decided to supplement the memory, bought a 2 GB DDR2 800 bar, installed it, and then dancing with a tambourine began ... The download began windows and after loading the logo, the computer went into reboot. I decided to pull out a new memory bar and leave it as it was, but now the computer has stopped booting from any devices at all, after the BIOS greeting, a black screen appears with a blinking cursor in the left corner and that's it. The BIOS correctly detects the hard disk, RAM, flash drive, floppy drive, but when booting from any media, nothing happens, only a black screen with a blinking cursor. I tried resetting the BIOS, connecting another PSU, and even transferring the processor, memory and hard drive to another known working motherboard, the result is the same. Comrades help with advice, I'm in stupor. I know that DDR2 is capricious and there were often problems with it, I just can’t determine the culprit, I think the problem is with the RAM, but I’m already starting to sin on the processor, because I don’t see any other reasons.

Answer 1

Everything turned out to be easier, the hard drive failed, the issue is closed, thanks to everyone who responded.

Answer 2

Follow the recommendations from the topic header
forum.ru-board.com/topic.cgi?forum=27&topic=21103#1
Then ask - there or here.

Answer 3

Perhaps, when manipulating the RAM, you damaged the mother ...