In a graph posted on verified twitter account @Bitstamp they show how (as of 11/2017) around 4.2 million BTC has been lost due to unsafe storing and wrong transactions.
Bitcoin is lost when trying to send it to incompatible addresses like Ethereum or other cryptocurrencies, Bitcoin can also be lost when you lose the access to your private key or the backup phrase. Bitcoin that has been lost can never be recovered. Bitstamp itself has been the victim of a hack in January of 2015 where they have admitted around 19,000 BTC has been lost which was worth around 5.1 million USD at that time (now 159.5 million USD)
In the tweet they had sent out they also pointed out that one of the safest ways to store your BTC (and other cryptocurrencies) is to use a hardware wallet like Trezor or Ledger and other hardware wallets are Keepkey and Digital Bitbox