Computer won't boot? Beeped 3 times on start up?

Hi all, I have a Lenovo Thinkcentre m91p with a few updated parts such as another 4g ram stick added and a Evga GeForce GT 730 but other than that every thing is standard. The other day my PC started up fine then kept freezing so I let it rest for a day and tried again, sadly it won't boot up but it will turn on and seem fine (The fans run and everything looks like it is working) but it shows no display on my monitor. I've taken out the ram and put it back in but no luck so far. Now it just beeps 3 times and shows no display. Please help