Is there any way you 'merely' unplugged the connector that goes to the screen? If the usual LEDs light up (and any usual POST Abeeps, and sounds of HDD seeking, assuming your HDD makes sound seeking...) then I'd wonder if this were the issue.
Although most laptops these days (IME) tend to just have a small RAM-accessing door just for that purpose, so as long as you hadn't disassembled the whole thing in the first place[1] I don't see any reason you'd have been in a position to do that except if it was half-unjiggled in the first place and in turning the thing over, perhaps moving the screen beyond normal 'open' degree, you managed to pop it off completely.
Worst case scenario may be that you managed to drop something conductive in the RAM-hatch (perhaps one of the screws you thought you needed to undo) that found itself at least temporarily crossing to rather unfortunate tracks.
I can't easily imagine the RAM itself is to blame (not if it physically fit the socket, even if it was slightly the wrong sub-spec) but if you're sure it was the act of fitting that delineated working and 'dead' (apart from fan, LED, and maybe other signs of life you missed... ethernet port flickers, eventually, if it has one?) then start looking at that.
Oh, and now you have disassembled it to get to the CMOS battery (a capacitative one?) so when you reassemble, look out for things you didn't unplug but look like they shouldn't be, perhaps. Maybe a good workshop manual is available for download to double-check it all, and shake the thing to check no mini-screws are rattling around in there.
(Covering a lot of bases, there, and there's probably as many more again I didn't cover. Nothing substitutes for being hands-on, though I don't personally relish repairing laptops in person, either, with their finicky and nigh on unique-every-time nature.)
[1] Are they starting to re-permanent the BIOS power, these days? They stopped that a long time ago in favour of either the button-battery clip as per desktop mobos or a fly-leaded mini cell-pack onto a jumper-socket, precisely to allow replacement/reset, though those going for the latter made them ultra-proprietry for the usual laptop "only Dell/Toshiba/whatever-authorise parts" reasons.