Dell mother board behaves abnormally

Dear Sir
I have a Dell Optiplex GX 280 system . Recently , there is no display comes - but the processor fan works normally and its speed increases gradually.

. i checked thoroughly. i turned the system off and removed the 4 wired connector (two yellow and two black )
and then simply turn on the system for 2 minutes and turned off
and then connected that 4 wired connector again
After i turned the system on.

no problem - display came.

i observed an important thing that this dell motherboard do not show the normal behavior of dell motherboard

that is when the system gets power the processor fan on the system rotates just for one minute and stops.

Here when the system gets power the processor fan on the system rotates and does not stop so no display comes

To get display i have to do the above mentioned procedure

One more thing i observed once the system turned on by using the above mentioned procedure and do not TURN OFF POWER COMPLETELY , at any time the system works .
Simply i can say the board light should always lights if it wants to get display

I request the kind hearted people to guide me to solve the problem

Thanking you