car wont start

I was helping my friend change the crankshaft position sensor which we did successfully ( I think)
Well after that we started the car and it was running fine then the owner of the car said that his alternator was going bad and pulled off the negative while the car was on but then it started to die so he put back on the cable and there were lots of spark and the car shut off and will not start back we check all the fuse and found one blown one that we replace but still no start. What to do now



Thanks in advance