Car wont start

the problems shouldn't be associated to the exhaust work unless king kong did the work , I would get underneath and look for any obvious problems ,scan for codes and get the definitions even if the light ain't on , find the brake fluid leak and take care of it once you get it started , test batt. for 12.5 volts make sure terminals are tight ,, see if that voltage is getting to the starter , ask the muffler shop to see if they noticed/had any problems

I had a similar problem turned out to be my fuel pump. If you have some one stand by the gas tank as you turn the key they should be able to hear the fuel pump engaging on and off. Hope this helps.