Display Problem

Ok, basically I've bought a new pc (not a pre built one, just components) as of last wednesday, me and my flatmates assembled it and got it all running but the cpu was severely overheating so we stopped using it and got a new cpu heatsink and thermal compound, whereupon we disassembled and reassembled it in order to install them.

After doing this the computer starts up but no display appears on the monitor. When using a vga cable it says check signal cable on the monitor, and when using dvi it does not, but either way there is no display. We have replaced both the graphics card and the motherboard and still it is not working. We don't think either of the new components are damaged although it is an unlikely possibility. We know the monitor works as we have tested it on another pc.

The hard drive still has the drivers for the old graphics card on it as we have obviously been unable to wipe it due to having no display, but I do not know if this will make a difference. Does anyone have any ideas on what it could possibly be?

All the power cables are connected correctly, and there is no display at all. Ie there is nothing when the computer starts up, no boot screen, no loading windows, no anything.

Can you confirm the PC successfully completes its post? do you have an internal speaker connected?

May seem like an opdd question nut my recent home build did not include an internal speaker, which i ended up needing to troubleshoot it.