iMac 7.1 not starting up - Black screen with 3 short, 3 long, 3 short bleeps

Hi folks, just wondering if anyone can help?

I have an old iMac 7.1 with the following specs;

Processor Name: Intel Core 2 Duo

Processor Speed: 2 GHz

No of Processors: 1

Total No. of Cores: 2

L2 Cache: 4MB

Memory: 4GB

Bus Speed: 800 MHz

Boot ROM Version: IM71.007A.800

SMC Version (system): 1.20f4

It was working fine and I updated it as far as it would go for my nephew to do his school work on... then after a few months it just would not start up. When you switch it on you can hear the fan and hard drive start up but there's just a black screen and it makes a few bleeping noises.

I've Googled these noises as they mean something but I can only find results for the opposite bleep pattern which has something to do with the memory slots. (the opposite pattern being 3 long, 3 short then 3 long).

The bleep pattern of my iMac is 3 short, 3 long and 3 short... this repeats after every 30 seconds or so?

Can anyone help me figure out what this is?

Thanks in advance!