|I'm assuming you DID NOT just install ram that was not installed in this computer before, and that the ram was working fine previously .|
If you DID do that, not all ram is compatible with using it in your mboard - if you still have the ran that was in it before that worked fine, remove the ram you just installed, and try using only the ram that was in it before
If this is a desktop computer,
remove the AC power to it,
remove the ram, wipe off it's contacts with a tissue or a soft cloth, don't touch the contacts with your fingers after that, install it again, making sure the notch(es) in the bottom of the module(s) line up with the bump(s) in the slots, and that the ram is all the way down in the ram slots and the latches at the end of the slots are against the ends of the modules,
make sure all cards in slots are all the way down in their slots and that the bracket on the one end is secured with a screw or otherwise.
While you're in there, if the cpu fan/heatsink has mung (dust, lint, etc.) on it, clean it off, but DO NOT use a vaccuum cleaner to do that (they produce a tremendous amount of static electricity when running, and anything connected to them can discharge that to your components) - use canned air, or an air nozzle if you have access to an air compressor, or an artist's brush that can be used in small spaces, etc. It may be difficult to clean the top of the heatsink under the cpu fan - the most likely place to have mung on it - and the bottom side of the cpu fan blades unless you remove the fan. If you have a case fan, clean that too if it needs it.
Also check for mung on the video card fan and heatsink if it has that, and the power supply's openings / fan.
With the cover still off, restore the AC power, start the computer and make sure the cpu fan spins - if it doesn't spin, if you're sure the power supply is working okay, don't use the computer until you have replaced it.
If it spins too slowly, and/or if it makes rattling or screeching noises, most likely to be noticed when the computer has cooled to room temp, has not been used for a while, and then is started up, the cpu fan's bearings are failing - the cpu is likely to overheat as a result of that if it can no longer spin it's full speed - replace it as soon as you can.
restore AC power, try to boot the computer.
If you have a video card in a slot, if it has a fan, make sure that it spins.
If the computer still does very little when you try to boot, this is the most likely thing......
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this:
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
It can be very hard to tell if your problem is cause by a defective power supply. If you have another working desktop computer that has the same wattage (output) capacity or a higher capacity , you could try the power supply from that with your computer, or if you can borrow one from someone else you could try that.
Or you could try your power supply with a working computer - it may not boot the other computer either.