Hi, @Meg ! Welcome!
If you’ve collapsed the simulator panel on the left, then there should be a control on that side of the screen to make it appear again. Screenshot below.
If you’re not seeing that, then feel free to include a screenshot of your screen so that we can all see what you’re seeing.
Also, going back to the MakeCode Arcade home page and then opening your project again should reset the simulator panel to make it visible again.