The beep is a POST code. It should tell you exactly what the problem is (in theory).
After a little googling, I found the k7n420 manual at
http://www.msi.com.tw/program/suppor...?UID=10&kind=1
However, that doesn't seem to mention anything on the POST codes (I just skimmed, though). You'll need to either look in the original documentation or give them a call and ask them. Hope you find it.