If I invest my money in a bank account, the bank could go bankrupt and I could lose all my money - well, at least I could have in the distant past, before government deposit insurance was invented.
If I invest my money in an share index tracker fund such as Blackrock's Aquila Life UK Equity Index Fund, I could lose all my money if all the companies forming the index become worthless at once, which is extremely unlikely - or if the mechanisms used by the index tracker provider (Blackrock) suddenly fail to track the index and instead track a zero line, which let's assume is just not going to happen.
Sure, if the stock market crashes, I could lose a lot of my money, but I shouldn't lose all my money.
So I think the only other way in which I could lose all my money invested in an index tracker would be if the index tracker provider itself went bankrupt. But how likely is this, and if it happened would I lose all my money?