You're doing a great job, it sounds like you're a lovely parent to her and have a great relationship. I think children can feel able to let their feelings out the most the person they feel safest, sometimes those feelings can be pretty intense.
12 is a funny age, young enough to still be completely overwhelmed by feelings, old enough to not always let an adult help you regulate anymore and to realise adults can't fix everything.
Maybe something terrible has happened, but maybe it's just teenage level terrible and it's an over-reaction. You can't know that yet, try not to worry too much about awful scenarios.
It's hard to know if she's locking herself away still because of how she feels about whatever originally went wrong, or because she feels ashamed and can't face up to what she did to you when she got home. If she's not usually like this, the latter probably plays a part in her locking herself away.
A note under the door saying you're not going to pressurise her to talk, you think she'd feel better for some food and a shower when she's ready and you don't have to discuss anything at all until she feels better, might be the way to get her out.
Above all else, if you're in-crisis, you need someone who appears calm and in-control.
A way to move it forward from the current position would be good. She sounds paralysed at the moment.
When we're stuck in an awful place the most helpful thing can be for someone to show us the first step down to de-escalate. You can deal with everything else (1. What the issue is which caused it; 2. Consequences to the behaviour when she got home), after she's able to calm down a bit.
So the first step is to get her out of the room and feeling a bit more human. Don't ask any questions about what happened or how she is, or ask anything from her. Just re-humanise and meet basic needs (food, shower, clean clothes). The rest can come in small steps after that. It would be good if her Dad can come and support you, but you need to be on the same page and calm.
Step-parents absolutely can parent. I've known some excellent step-parents who have great relationships. You sound like you're doing a great job in a really hard situation and it's great to see you put your step-daughters welfare on the same level as your own children. Your post on evaluating the needs of your kids as being one who is in crisis and the other two being completely fine is exactly that, you're treating them as equally loved and cared for.
Keep at it, you're doing great. I'm the birth-parent of my children and I'd be finding this just as hard as you. We all rely on the strength of our relationships when we need our kids to talk to us. It sounds like you've many years of good relationship to draw on.