Quick, Help! How did my Sim get stuck under the stairs and how do I get her out?

Silly me. Once I'd posted, I realised I could knock out a wall in build mode and then rebuild. :(
Feel free to share your D'Oh moments though.

Just for future reference, this will happen when you go into build mode and start working and a sim is on or almost on the stairs. Before I learned this, it was driving me crazy by all the sims who kept getting in that situation.

The problem is when this happens with pets, for sims you can enable testing cheats and shift click on the floor "teleport here"

Yer a wizard Harry

Thanks Folks.
Welcome to the forums @Flexono1

I usually jump into build mode and grab an archway, then once the sim uses it to get out I can quickly delete it.

This happens at least once a game for me on Eliza Pancake. Not sure why she likes to play Harry Potter so much but she does! I finally deleted the walls around the bottom of the stairwell. I'd rather they not look pretty and walled in if I have to ctrl+shift+c all the darn time to teleport her out. :)

I keep finding pets trapped behind stairs when my sims return home from a different lot.

Don't feel bad I had to come to these forums for the same thing not that long ago. Eventually, I removed the staircase to get my stuck Sim out. Replaced said stairs and recently, when the wife got stuck there, too, I placed an archway into the Livingroom.

In this house you could prevent it from happening by reducing the stair size from 3 to 1 tiles (at least I'm pretty sure it's 3 tiles wide atm). Takes a bit of remodeling, but will give your sim a safe place to teleport when reset while on the stairs. If you want to be extra sure, you can, in addition, knock down the thin wall that is part of the stairs.
I'd add pictures but I'm on mobile and not near my game right now.