Exhaust header leak or worse
 

I noticed a little puff of exhaust gas from the front left header when I was sat on her at a petrol station. Rode homewards and after another five miles I noticed she was a little sluggish then she blew. Sounded as though the exhaust had been removed. Noise was definitely coming from the forward end of the bike.
There was an instant and complete loss of power and the engine cut out. Now she won't start. Doesn't seem to be leaking fluid anywhere

Any suggestions what it might be, have 0 mechanical knowledge but would rather fix her at home if it is just a gasket or something easy.

Hope someone can help