Taking a close look at the adapter, you will notice there is evidence of melted metal on the second prong most likely caused by arcing.
What this tells me is that there was a short from the hot to neutral which (should have) tripped the breaker. What caused it? Any number of things from faulty wiring to a surge with enough voltage to cause the electricity to "jump" from one conductor to the next.
Is your adapter safe to use?
Yes, provided it works. Adapters are designed to isolate the high(er) voltage mains AC from the lower, cleaner, usable DC voltage. If anything was damaged due to a surge, the adapter simply won't work (most likely the internal fuse would be blown).
Bottom line... you can plug it in and it won't hurt your Mac. You can even used the charred AC plug connector and it will still work. Unless the prong is broken, or so physically deformed that it's causing a short, there's nothing to worry about.