are you suppose to disconnect the battery before welding

Yes,it will short out your computer and sensor,s.

While there is disagreement on the forums I looked at, some say yes, some not necessary, personally I have had exhaust work done on a car with a MIG welder, (not acetylene), a trailer hitch installed on my truck with an wand arc welder, and other things and never disconnected the battery, and never had a problem. BUT!! I am talking about OLD vehicles, without all the sophisticated crap on modern cars, and for peace of mind it's probably a good idea. ......Is the reason you ask because you had something welded and now the car does not start? Is that how your two questions are related?

TST and I were typing at the same time. For the record, he knows, you can trust his answer

Maybe I should have added that it is a 1997 Saab SE Turbo. And, it does not start, nor, do the power windows operate. The mechanic told me he does 20 a day and NEVER disconnects the battery. Other mechanics say that it should be disconnected, which makes sense to me.

20 a day what? Exhaust work? If it ran fine and windows worked when you drove it there, then would not start to leave, I don't believe in coincidences like that. You question really got me curious and I called a friend that has an auto-repair shop, he does work on exhaust everyday. He put a cat and muffler on a 1990 Lincoln I had at the time. He told me he does not disconnect batteries,. but only uses a acetylene and brazing rod,