Computer wont turn on completely

I have an Apex computer, p3. Recently i had to take the ram out of the motherboard to see what style they were b/c it was impossible to find out without removing them, they had no stickers. I also had to temporarily remove the video card to get to them. its a tight fit. I put them back in all the way, locked them in, and now when i turn my computer on, the only thing that happens is that the red light comes on. the green light does not, my monitor, which is in working order, is on but never picks up any video feed from the computer. is there something i have to do with the ram specifically to get it to work again? I tried the video card on another computer and it does work so i know it is not that.