Why did my trade close without me doing anything?

This was most likely caused by a 'Margin Call'. A margin call is when the available funds you have to trade (Available Margin) has dropped to a level which has triggered the closure of your open trades.
You can see how much margin you have on the positions page...

The level this trigger is reached is typically when 'used margin' reaches 50% of total margin.
By way of warning, you will normally receive emails that margin calls are approaching. To avoid having your trades closed, you will need to add funds to your account.

Another possibility is that your drawdown has reached the "hard stop" level.

When this happens all of your trades close to minimise potential losses. If you don't know what a Max Drawdown is, or how to reset it to carry on trading, please refer to articles: "What is Max Drawdown?" and "What does it mean for someone to "Reset" their "Max Drawdown"?"