Memory slot stopped working- 3 beeps on startup

Hi

I have an I-mac early 2007 20" 10.8.5 2ghz core 2 duo

Suddenly one of my memory chips has stopped working... it has all been working fine for ages until the other week... Memory made by Crucial

When both memory chips are in both slots and i turn the power on i get 3 beeps that repeat until i turn it off

If i have one chip in the right slot it works (with half the memory!)

The other chip also works in that slot -

- So it seems my one memory slot has failed?

Is there anything else I can do to see if this is the case? Is it common? Is it fixable? What are my options?

Although it's getting old it still did a good job up until this happened... Id love a new one but cant afford atm!