been using cube as ext. HD, now it wont boot!?

I have been using my cube as an external hard drive to a power g4. Did this a few times, and actually once I started it up without pushing down the "t"...it continued working fine though, now I got a monitor and tryed to start it up by itself and it freezes in the blue screen style...??

Does anyone know what I'm talking about? apreciate some help thanks.