Sounds like the LCS has failed (ie. pump, liquid dried up, leaking) based on your past reports of high fan speeds. Try lying the G5 on it's side, the liquid might have dried up and this might move around what is left in the tubes to get it going for a little while. Also remove "G5" cover and check for leaks. Ideally you need to rebuild the LCS and you're pretty lucky it has stayed running up to now with heavy use.

Also double check RAM placement - matched going "outwards" in the slots (starting in the middle)