What does the loud beeping mean when I plug my computer in mean?

[CJM629]

I have had my computer unplugged for a few months, I’m now trying to plug it in but as soon as I hit the power button it does 2 loud bold beeps and the 4 light beeps..and nothing comes on the screen..help plleaseeee

[Fanordi]

Hello, if I check HP documents, this error appears to be a BIOS problem.. 

So if you don't have tried to modify your BIOS, no BIOS update, just unplugged your PC during a long time, it is possible your CMOS battery is dead. 
I recommend to you unplug your PC, and open it to eject your CMOS battery, and wait few times. After, replug your PC and try to boot with any CMOS battery. 

The CMOS battery appears to this: 

For opening maybe this manual can help you: 

http://h10032.www1.hp.com/ctg/Manual/c05145971