How do I troubleshoot after hard drive and ram replacement?

I've just swapped out a 120 gb hd for a 750 gb hd & simultaneously replaced my 2 gb ram with 8gb ram (2 4gb). My mac mini won't start. I get power but nothing at all on the screen.

Thanks to all for this feedback. I've replaced the old RAM, and in so doing discovered two things:

1) the orange band connecting the bottom housing to the internal framework was attached only to the internal framework. I'd left it unattached to the bottom housing because none of the disassembly tutorials I consulted mentioned disconnecting it or reconnecting it, and when I separated the two halves to get access to the RAM and HD, I didn't recall having disconnected it from anything. However, this time, since I was able to find something to connect it to, I did make this connection.

2) the top of the two RAM chips was loose. In succession, I tried putting the 4gb chips back in place & reassembling, then removing these & replacing them with the original 1gb chips.

Each time, I got the same result: if the orange ribbon cable was connected, the drive would turn on without booting but would make a low-pitched beeping sound at approx. 3 second intervals. Nothing on screen at all. I tried disconnecting the orange thing again with the original RAM in place -- got nothing at all except the power light. E.g. back to square one.

On the theory that it would make most sense to retrace my steps from the beginning, I went back & reinstalled the original 120gb HD as well (and in the meanwhile tested the 750gb HD by plugging it in via USB to my laptop -- the new HD is fine).

Even after reinstalling everything as it had been I'm stuck with a mac mini that won't boot, won't do anything but give me these low beeping tones. Any other thoughts or suggestions as to what I've somehow managed to bollocks-up would be most welcome!

ajr