It means that your RAM is either;

1) Faulty
2) Slightly un-plugged

How do you feel about opening the PC?

Since I've had it (about 3 years) it's had so many problems it's been opened a lot. In fact its not screwed on because I got tired of having to unscrew it everytime.

It hasn't been opened for a while and in order to get the light on it I'll have to unplug everything from the back to pull it out from under my desk but if it's necessary I can do it.

Good, then take a look at the RAM and if necessary, take the RAM out of its slots, clean the connectors with a soft cloth and then re-seat it.

A BIG BIG THANK YOU! I took out the RAM and wiped it and now its working again.

Nice one i've had this problem before, glad you've got it resolved marked as fixed ,

HAH! A pleasure my friend Glad to have helped.