DNA Revelation

I'm one if those DNA stories you hear about. Long story short: I did an Ancestry test about 10 years ago. My sister was browsing around Ancestry about a year ago and discovered that I am suddenly only 1/2 related to my entire family, and there are all kinds of names on my paternal side that I have never heard before. Im 55 years old and just found out that I'm not biologically related to my dad, who passed away 2 years ago. My mom insists there was a mistake, so I did another test and the result was the same. Mom is no help, so I'm left to figure this out on my own. I've uploaded my ancestry test to GEDMatch, and Ive emailed the few people at the top of my paternal side. What should I do next? I would like to know who the guy is, and maybe even meet him, but he's got to be in his 80s so there's not a lot of time. I'm open to any advice or direction you want to offer.