I can't answer your question specifically because I'm in the U.S. and this is about a Canadian ETF and a Canadian account. But I can surmise what the reason might be based on what happens here in the U.S.
I suspect that today is the ex-dividend date for your ETF and that there was a distribution. When that occurs, the previous closing price is reduced by the amount of the dividend. Because of this, open orders must also be adjusted by the amount of the dividend unless the investor has attached a "Do Not Reduce" condition.
If your stop order has been cancelled rather than adjusted, that may be your broker's policy for open orders that exist on the ex-div date.
Either way, you should contact your broker to ferret out the details.