Sometime in mid 2013 or 2014 as a junior dev I woke up to a call from my company's CEO. He informed me that the legacy system they use for order processing is down nationwide that nobody can add new orders until it's fixed and that I needed to fix it. I had been working there 6 months and was hired along with a senior dev to begin developing a web app to replace this legacy system. The senior dev had left the company two weeks earlier for a better offer so it was put on me to figure it out. I was very frank with the CEO and told him I didn't know if I could fix it and suggested he try to call the company they hired to create it. I didn't even know where the source code was let alone what the design paradigm was or whether or not there was any documentation. He said he would try figuring out who created it and give them a call and asked "As a developer you shouldn't you be able to fix this?" I just told him it wasn't that simple and left it at that.

I get to work and the CEO has discovered that the company who created the software no longer exists and I tell him he may need to find a company to consult on this if I can find the source code and if I can't find the code he might be screwed.
I found the source code in a random IT shared folder there is no source control, no documentation, no unit tests, no test environment, and it looks like nobody had touched it since 2005 or about 8 years.

Despite being completely unfamiliar with the code and the design paradigm I was able to figure out that they were validating customer addresses against an old Google geocoding API that was shutdown the day before and the lack of response was killing the application. I fixed the issue and warned the CEO before deployment that I wasn't able to test but he said to go ahead and thankfully all went well.