my ram memory decreased from 512MB to 256MB when i changed my hard drive, what shall i do?

Where are you reading this figure, System Properties, or System Information. If it is under System Properties the figure that you are reading isn't you total physical memory like you will find in the System Information.

Check to make sure that the modules are properly seated, they could have come loose while you were working in there earlier.

both, in system properties and in system information.

so ,as suggested by other poster did you try removing the ram sticks and re-seating them

yes I did, nothing happend

ok ,so changing a harddrive will not affect the ram count ,so one stick stopped working ,remove one stick and boot computer ,if computer boots ,then remove and replace ram with the other stick ,if computer boots ,then you know both stick work ,so that leads me to think that they don't work together,seen it many time ,if they did work together before them i'm at a lost for a reason ,sorry

Actually I don't understand what shall you do (Ahmad). whats your problem?