Do you know what BIOS you have? Each BIOS is a little different regarding beep codes. Find out which BIOS you have, and google search that BIOS beep codes and you should find what you need.
From the short research i found i couldn't find anything with one long beep by itself. Unless you mean a continuous beep; "Continuous Beep No Power, Loose Card, or Short." For IBM BIOS. Could also be RAM, check to see if it's seated properly. Though usually RAM is 2 short beeps.