Anti freeze cap broke :(

So, it is extreme temperatures here in Toronto, and hindsight is 20/20... but I was trying to fill up my anti freeze with DexCool this time (since last few times I had just topped off with water), and I'm getting low coolant warnings in my car.

I tried to twist it, and then the yellow cap broke. What should I do next? Can I buy another cap? Can I pry off the existing thing that is still stuck in the reservoir? Do I need to take this to a mechanic? A dealer?

What are the implications with little or no coolant while driving in the cold Toronto weather right now?

Any help is appreciated.

Thanks

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