a quick check which will give an easy diagnosis is to get a voltmeter and stick it accross ur power and ground connections. This will tell u if you are getting power to ur amp. if you get no voltage, then yeh one of these cables is loose and from experience, its probably the ground cable not making good contact cos its pretty hard to mess up running a cable from the battery into the amp. If thats alright, then it may be your speaker wires, if all speakers attached are turning off, there may be a short in one of the speaker connections causing the amp to go into protection, but this will usually light up some sort of light on your amp, so you could check that out too.

I reckon you should have a go at diagnosing and fixing it yourself. That way u will save urself a buck and learn a thing or two in the process and ull know where to look next time. If you search this site as well as a couple of other car audio forums (such as www.mobileelectronics.com.au) you will find mountains and mountains of information that will help u out.

otherwise yeh...im sure therell be plenty of people on here who would be willing to fix it for you