Hello and welcome!
Check if the full 1GB is recognized in the machine's BIOS. The fact that it worked fine
last month says very little about the
current working state
Running a memory test from
http://www.memtest.org also never hurts.
Also make sure you've installed the memory modules in the correct position and that these are the correct memory modules for this board.
Check your manual and
assume nothing, this has bitten me quite a few times unfortunately where I was too lazy to check the manual.
In my experience it is always best to make 100% sure your hardware is fine first when there is even the slightest doubt if may not be OK, and then move on to the software when you've checked the hardware.