Theymos once answered on a thread on the same topic:
It was a DDoS attack.
I don't know what the reason is for the current one, but I totally missed it and didn't see it. But thankfully, everything is back to normal for now. Based on my quick search, this bad gateway is nothing new to the forum as it has happened dozens of times in the past. I found a lot of threads, but this one might be a good one.
Welcome Back 502 - irfan_pak10