No boot, cpu still running arrgh

I disassembled my old pc (7 yrs old) and cleaned the case and put it back and ever since, it hasn't booted.
I don't know the mobo no. since I lost the manual.

The problem is that everything in the case keeps running (it seems to) but nothing is displayed on the screen.
I tried all the tricks, checked the ram etc...

The pc has a graphics card as well as internal vga.I tried both.I even tried all the bios cleaning tips like removing the battery for 10 min, changing jumpers below it etc.yet nothing happens.

Also I removed the both the RAM chips and everything else but cpu psu and mobo. and booted and there were no beeps !!!!

Specs
Intel P4 1.7 Ghz
DVD r
HDD 40 GB Seagate
5200Fx nvidia graphics card
768MB DDR1 ram (256 X1 and 512 X1)

Yes actually there is an onboard speaker (little small black one) and a big one on the case.
Both connected.

Does the mobo have a 4 pin CPU power? If yes, make sure it's plugged in all the way.

Any fans spinning?

I'd try a different PSU.

The presence of power does not mean the power is stable enough to run the computer. My best guess is still the board though.

sorry for being clumsy in my replies.

How can I check which one is bad ??