Sony Amp in an A3, worked.....now doesn't

Hello all, just wondering if any of you guys can help me, i have an audi symphony 2 head unit, i think thats the double din one, and i not so long ago had a sony amp plus 2 12" sony xplodes put in the boot. I payed a bloke to do it and it all worked fine until suddenly it stopped working. I checked all the fuses, all ok, I checked all the leads and they look fine, the only thing i can suspect is the remote low voltage connection from the headunit to the amp. I just got a set of head unit keys and got the thing out and it looked like it was connected ok. So why isn't the amp turning on? It the Sony XM-D1000P5 amp, are these known to go wrong?

Any help you guys can give me would very much appreciated!

Ah, good idea with the multimeter!
I had a look at the fuse next to the battery as your said and it looked fine. Do you know anywhere i could get a mulitmeter? Its something that would actually be very usefull so do u know anywhere i could get a good one? Thanks very much for your advice!