Why is my 50watt Bentley amp making loud sound when switched on?

I had a gig last Friday and turned my 50watt Bentley (75watt mains) amp on and it produced an ear-splitting loud noise. I turned all the volume, levels and dials to zero and it still produces the noise when I turn it on. There was no bang, smoke or sparks, just this awful loud scream! I plug it directly into the mains and have nothing else apart from my bass going into or out of it. I have not had this problem with it at any other gigs, and it's only 3 months old! I have gigged at the venue before so I know it can't be there power, nothing else went down just my amp! Can anyone help me! Thanks!!