The novorapid will stay in your system for 4-5 hours from when you took it. Best to make sure you've had enough carbs to cover the Novorapid dose. I think I would wait a while before taking Levemir - when is your next Levemir dose due?
oh dear I've done that too. You'll need the quick acting carbs as others have suggested. I've used my bolus instead of basal before and only noticed once I had already injected.
So the Novorapid has probably done about 70-80% of it's action by now. It depends how confident you are that it won't take you much lower in the next 2-3 hours. What are your BG levels like now?