I have not heard of "to strike rich". The usual expression is "to strike it rich". He struck it rich when Microsoft bought his software development business for $10m..

You can make a 'rich strike' which comes from the mining industry when they find a rich seam of coal or gold etc. Or, 'you strike it rich', which would mean you have been successful in some way or have made lots of money. An unexpected discovery could be anything, for example the drug 'penicillin' was found by accident, they were looking for something else at the time, so it was an unexpected discovery, however, they 'struck it rich' when they found it, because it was so useful.