Not sure the Beatles really count - as soon as 'Free As A Bird' was announced it was obvious it would become a hit. And they'd had a hit earlier in 1995 anyway.

A lot of obvious candidates already cited here I see. I also think it'd be fair to rule-out posthumous comebacks after long hiatuses as these aren't strictly 'organic'. So that sets-aside people like Elvis, Roy Orbison, even Jacko.

 

The first act I thought of outside of these was Texas - by 1992 they appeared to have been past their moment in the commercial sun, such as it ever had been with a sole Top 10 single in 1989. The last thing I expected was that an album released five years later would yield a slew of Top 10 singles, and a far-superior run of hits from 1997 to 2001 than they'd ever managed in their first few years.