No signal on screen with Alienware x51

 So this all started when I took my hard drive out of my Alienware X51 and put it into a sabrent  hard drive dock and plug it into the same desktop to see if I could boot off of it.  So I turned on the computer and it shut down to select the different boot priority and it turned back on and tried to boot off of it but it didn't it said there was an error and said it could not find a boot device and then kept restarting and doing the same thing so then I just turned it off and turned it back on and then it then the screen said it had no signal.  I took the hard drive out of the hard drive dock and put it back in the computer and it still did the same thing I used another hard drive out of my laptop that have done before and it worked but this time it didn't.  I will tell you exactly what the monitor does when it's not plugged into the computer it waits for five minutes till it automatically shuts off but when I plug it back into the computer it says there is no signal and it goes into Powersave mode when I turn on the computer the monitors still does not show anything. I have tried so many things so I don't know what else to do.