Presario 900 i turned it on an no display power

Probably itz either minor case of software corruption/improper ground/cpu fan..
or major case of faulty or defective main board/ram/daughter or addon card(s)..

..plz do check the following as in order:..
the Hardware part:..
1)a) in power-on mode, open/unplug the data/vga cable of the monitor from cabinet..see if raster or 'osd' is appearing on the monitor screen..if so monitor is ok..else replace the monitor and see..[not applicable in case of laptop]..
b) in case of laptop try to get output from external display unit/monitor using vga port..if there's proper display in there..the problem is probably with either the 'lcd' or 'ccfl' or 'inverter circuit'..

2) if u hv a graphics card, replace with a standby or onboard and see..[not applicable in case of laptop]..

3) unplug mains supply..pick open/remove the cmos battery..check (above +3v)..resit..

4) see if the ram(s) is loose/faulty..open clean/remove carbon & or oxide formed on the teeth with an eraser n resit it..or replace with a standby n see..

5) a) see if ur hdd is noisy or emitting any 'ticking' noise (like 'tuk tuk' or 'kirr kirr')..
b) or if the system is not supporting either the newly installed device or itz software/driver /firmware
if so, u need either a fresh os installation or ur hdd is likely to b replaced..

6)remove/clean dust neatly..dust is non-uniform in nature n may cause jam/short circuitry/excess resistance n many more..resulting an increase in heat..

7)open the CPU Heat-Sink with fan out of the socket..put some "Heat Sink Compound" (available in local electronics chip level spare market)between the cpu/processor and the CPU Heat Sink *** fan..resit/fit it tightly in position..

8) on main/mother board..see with a debug card for mb error..may b short..look for defective capacitors(espabove1000mfd)/mosfets..go through the mb's south bridge n the circuit lie around atx connector..the crystal clock generator (if cpu isn't heating up)..

9) [in case of desktop] the smps itself may b defective/faulty..check the outputs of smps viz. 0v (black~ground); +12v(yellow); +5v (red, green~ps-on, violet~pg& grey~sb); -12v (blue);-5v(white); +3.3v(orange)..the 'oscillator and error amplifier ic',or pwm switching regulator circuit..look for defective protector-transformer/resistance (esp 4.7k)/mosfet..
check the battery n adapter [main power], in case of laptops..

if u r not a professional, always better to seek advise from ur system admin/vendor....