You might have damaged the RAM or the RAM slot with Electro-static Discharge.
To avoid this, ground yourself simply by touching the chasis of the computer and keep a connection to it before you touch anything inside your computer.
After you are grounded, move the 1gb working RAM stick from the working slot that it is in, to the non-working slot the 256 stick was in. Does it work with just that one slot?
Be aware that some older motherboards require that at least two sticks are installed for pairing, in order to work, while others are even more picky and require all RAM slots to be filled, either with actual RAM or a blank RAM stick to "trick" or satisfy the computer.
We need to know the following:
- Post the Model of the 1gb stick that is working, that you have installed in your system.
- What motherboard do you have?
- Did putting the 1gb working RAM stick in the other slot work?
You must match the RAMs specs very closely or exactly for it to work.
RAM that is 1600mhz\PC128000 will not work with RAM that is 1333mhz\PC10664.
If You have the same speed (1600mhz) but one stick is 1.5volts and the other is 1.25 or 1.65, the RAM will not work, and will burnout the lower rated RAM.