really bad that you lost much amount of money in btc but seems you must did some mistakes to use online wallet
I used block chain wallet from 6 years and still not got any issue
as mostly keep small parts of coins in many different wallet ids and never keep all coins in one wallet
moreover never download any suspicious software or male-ware etc in that pc using for block chain wallet
may be you received some fake email from block chain and you click on link inside email which take you to fake block chain website and as soon you put your password in fake website, scammer got info and just able to transferred all your coins
online wallet can be use in safe way but always be careful and never download any files or malware or never use NON secure browser and always keep coins in parts in many different wallets