A while ago, I bought some Kingston RAM (HyperX, DDR2, 2x2GB, PC6400). Since I was under 32 bit XP, and only ~3.4GB was accessible, I upgraded to 7. However (I'm not sure exactly when it first happened), I now notice that only 2GB of RAM is visible. CPU-Z says that only the second module is there. My motherboard is a Gigabyte MA790X UD4P, and processor AMD Phenom II X4 940.
1. Bad module, though I have no theories how it suddenly went bad, but still possible.
2. Not seated correctly. Did it slip out by chance?
3. Bad slot on mobo. Try running your dual channel setup in the other two slots.
4. Clearing CMOS may help.
5. Ensure RAM settings are off of "Auto" and are at manufacturers specifications.
6. Run MemTest86+. Download it from memtest.org
Normally, memtest is the first suggestion anyone would make, but seeing as it was recognized in XP, it stands to reason that it should be working now and running memtest is a long, long process. If you do run memtest, run it all day or overnight.