If you utilize the two factor authentication with blockchain.info and take pre-cautions, like install a browser specificly for blockchain.info, and don't use it to visit any other site, it's almost impossible to lose your coins, unless you done something incredibly stupid. Blockchain.info don't actually hold your coins, and can't actually access your coins without your password. The only other possibility, is if blockchain.info's server got hacked and source code got changed, it's pretty rare these days for a competent website to lose control of their source code, I think I've only heard of this happening in the case of the employee of the hosting provider has done the hacking. If blockchain.info's source code got changed, then all users who login during the period can potentially lose their coins, until piuk the operator discover the hack. Again this is an extremely rare case, and would not affect you if you did not login during the time of the hack.