Try just the 1 stick of RAM in the first slot...

Were you trying to flash a new BIOS when you got the CHECKSUM or did it just die all off a sudden?

I put 1Go RAM on 1st slot (near CPU) put the jumper to clear cmos, left the battery ou for about 30 min.

Put everything back the way it was.

Turned on the PSU, press the on button, the green led is lit, the CPU FAN is on, the Chassis Fan is on, the power fan is off. The monitor is in standby mode waiting for signal.

Can anyone please help me on this?

Sorry I must have missed the last reply somehow.... If the BIOS comes up with a CHECKSUM error its more than likely corrupt. If inserting the motherboard CD to revert back to its original BIOS doesnt work its probably time to RMA the board I'm afraid.

You did put the CLR CMOS jumper back to its original jumper didnt you??

Paul is right. If the fans are working, the PSU is probably fine. Most likely the BIOS or other problem on the mobo.

nothing comes on the screen anymore

it stays offline

Most likely a BIOS corruption.. If the PC is unstable and you try to flash a new BIOS there is a high risk of it not being able to finish, thus corrupting itself...

As a last ditch you could try 2 sticks of RAM in slots 2 and 4, then putting in the motherboard CD to see if it will 'suck' the default BIOS off the disc..

If it doesn't work, will I have to buy a new motherboard?

Try returning the board you have to Asus if still under warranty (typically 3 years). This process is referred to as "RMA"'ing a board (i.e., a return with manufacturer's authorization). If it's out of warranty you will have to purchase a new motherboard.

It also depends what country youre in. Here in Australia (where I am) Asus doesnt take back motherboards for RMA...They have to go back to the retailer, then the distributor for repair. A long drawn out procedure designed to make you angry..

Its a fact of life that sometimes these boards just up and die all of a sudden... In your case the crashes may have been an indication that it was dying...Sorry mate