That is impossible using simple mining rig. I am doubtful if the amount you mined is 50 btc or maybe it is not 50 btc after all or other cryptocurrency or just another amount. It could be 50 mbtc or it could be 50 coins of any altcoins. I do hope you can provide us with a screenshot so we can really tell if the is really 50 bitcoins. But if that is really true then you are very lucky and that 50 bitcoin may not be coming from your mining but rather it maybe coming from somebody else. Or possibly somebody has made an error and sent you that huge amount. Well goodluck and congrats, you dont need to be puzzled anymore if that is really btc try encashing it and enjoy your profit.