exhaust leak?

[slowcivic123]

So I have a 93 civic lx and I just recently changed my upstream O2 sensor and now I have an exhaust leak from my exhaust manifold. I don't really see how that happend but i snapped my first O2 sensor off while trying to replace it so I had to saw the rest of it off, could little pieces falling into the exhaust cause this? I've never really done any exhaust work so I don't know where to start, I was debating on just getting some seaform at first to clean everything out and see what happens. If someone could help me out here I would appreciate it.