DIMM slot not recognising RAM

[Punkrock-101]

My 2nd DIMM slot is not recognising my DDR2 RAM.

I have 2 slots with 256mb in each. I safely took the 2nd one out to check the type because i am upgrading to 2gb. I put it back in again and now when I press F2 on startup the 2nd DIMM slot is classed as "Empty"

Any suggestions? Thanks

 

[Tedster]

reset your cmos and retry. If that doesn't work - your slot went bad.

 

[Punkrock-101]

What is CMOS and how do i reset it?

I know the slot is not completely bad because when I start up the system with the memory in the 2nd slot it just doesn't recognise it but when i start it up without it in it jus goes wierd and nothing is displayed on the monitor.