Electric hot water system making cold water

Greetings,

I have an electric hot water system - the type with a big tank sitting up in the roof.

Suddenly, it's not making hot water. I've checked the appropriate switches on the mains board and they're both "On".

I've been up in the roof, and it is refilling itself properly - it's just not heating up the water. I'd heard these systems kind of go on forever, so I'm wondering if there's something else I can check.

I've called the plumber - waiting for a call back - but would love to avoid the fees if I can sort it out myself.

But, it seems to me like it's just the electrics stuffed up. In which case, I won't be fixing anything.

Any thoughts?

Thanks.

PS - probably a coincidence, but a couple of days ago, we had brown water coming out of all our taps - hot and cold. It took several buckets full of brown water until it eventually cleared up. That followed a day of 42 degree heat!