There are BIOS setting which can disable power button and decide the state of the server when power cable connected. You can check those settings to see whether power button is disabled. You can refer "System Security Screen" section on page 48 of below link for more details on these BIOS settings. "Power Button" and "AC Power Recovery" are the 2 setting which you need to check [URL]   
I just got a used/refub server to help me understand enterprise training better. I got a Power-edge R900, I know its older but I still want to learn more. I come from the home small business side and need more server training. upon booting up the first time ran into minor problems because items needed to be re-seated after shipping. I did all that and booted up. went to bios and checked everything, told it to restart, it did and I even setup a raid with 2 drives. However, after getting everything configured, I am waiting on my OS CD to arrive and I decided to just shut the server off till it gets here, but.... The power button does not shut the server off...??? I even held it in for 30 seconds, then a minute. ..? nothing. I pulled both power cords to manual disconnect, then re-plugged them, system came on immediately and booted. I tried again with the power button, it refuses to shut down???? I read half the manual but everything in there says the power button will shut the server off. Any Ideas?  
R900 Server does not shut off with power button.