After reading many articles and Wikipedia pages, this is my best attempt at a summary of the whole story:

Bulgarian Ruja Ignatova spent over a year in prison in 2012 for a fraudulent scheme she ran with her father, and then went on to run an MLM scam in 2013 called BigCoin, before finally founding OneCoin in 2014.

OneCoin was a classical Ponzi scheme in that it paid out old investors with the money of new investors. There was no actual blockchain behind OneCoin, but they pretended there was by simulating transactions in a sham database and claiming there were miners validating transactions when there were not.

Suspicions grew around the world from 2015 onward, eventually leading to investigations and arrests in several countries. In October 2017, Ignatova disappeared after seemingly being tipped off about her impending arrest. She left her brother Konstantin at the helm when she fled, and he was arrested in 2019 and is facing a maximum of 90 years. In China alone 98 people were arrested and prosecuted, with over a quarter billion dollars being recovered.

When the scam fell apart in 2017, it is believed they had stolen around $4 billion USD from investors worldwide. The Times described it as "one of the biggest scams in history". Estimates of how much Ruja herself made off with range from half a billion up to the majority of the full $4 billion.

Bulgarian investigative reporters have claimed she was murdered and dismembered in 2018 aboard a yacht and thrown into the Ionian Sea by a Bulgarian drug lord who was apparently trying to cover up his own involvement in the OneCoin scam. However, the FBI are apparently still operating under the assumption that Ruja is alive.

In the summer of 2022, she was put on both Europe's Most Wanted list and the FBI's Ten Most Wanted list. She is also the subject of an Interpol warrant. The FBI are offering $250,000 for information leading to her arrest.

In January of this year, a property in London went up for sale under her name, leading to speculation that she is still alive. Some investigators have pointed out that if she is, she has probably had plastic surgery.

Ruja was the subject of a 2019 BBC podcast call The Missing Cryptoqueen, as well as a 2022 book of the same name.