“You can not impeach someone who is not holding office.”
It’s been done before.
General William Belknap was impeached (unanimously by the House) for bribery in 1876, AFTER he resigned. He was later acquitted by the Senate, where some members felt his resignation prevented them from having jurisdiction. But the point is open to debate.
Impeachment is typically used to remove someone from office, but it can also be used to cancel pensions, prevent future office holding, and suspend licenses like in Bill Clinton’s case. (He was fined and had his license to practice law in Arkansas suspended for 5 years, for perjury.)
Being out of office does not, to the best of my knowledge, prevent a civil or criminal trial. As for Belknap’s impeachment, it was an investigation run by Democratic Rep. Hiester Clymer’s Committee into Belknap’s alleged financial dealings. Many at the time said Clymer overreached (typical of a Dim). Interestingly, Belknap was a Democratic Republican and would be considered a conservative Republican today. The more things change, the more they stay the same :)