dell optiplex 755 will not boot unless it is in service mode

a friend of mine gave me a motherboard of a dell optiplex 755 along with the power supply, power switch, and fan, when i first plugged it in the fan and hard drive spun up but there the screen was blank and there was no beep from the motherboard, i tinkered with it for a while, tried plugging things in and out or replacing them, it was until i moved the CMOS jumper to service mode and it posted but if i put the jumper back in to the PSWD slot i won't even after replacing the CMOS battery, what could be causing this?