This is a great teachable moment, both for your daughter and the other child.
First - talk with her about the risks of bringing things she cares about to school. I would usually not recommend having kids take things they care about to school - not because I think the other kids will be bad per se, but it's just too likely to lose things, accidentally break things, etc. But - it's her choice, for the most part; obviously with your limits preventing her from taking something inappropriate or too expensive to risk. Don't blame her for this happening - it's not her fault! But, she should keep this in mind when making choices in the future.
Second - she should talk to the child tomorrow, and ask her if she brought it back. If not, she should ask if the other child can please bring it back. This is the first step - as it would be for an adult, too!
If it doesn't come back the next day, then it would be appropriate to ask the parents of the child if they could help her out. Don't be confrontational - after all there's a good chance this was accidental, and even if it wasn't, there's no real value in being confrontational. Just ask if they can help her remember to bring back your daughter's toy that she accidentally took home.
If you don't know the parents, you can ask the teacher to help you contact them, or maybe talk to them at pickup if that's possible. Again, not confrontationally - just fixing the problem.
There's a nonzero chance though that the toy is lost - the child may have brought it home and then lost track of it, or lost it in the car or who knows where - it's something that happens with children. If the parents aren't able, or willing, to help you, then you may have to make your peace with the loss, and help your daughter do the same.
One side note: "Pop-its" are very commonly used with children with autism or sensory disorders. If this is why your daughter brings it with her - if it's basically a therapeutic device - this changes things slightly. In that case I would still have her ask the child, but I would also probably involve the teacher - not necessarily to retrieve it, but to watch and help your daughter out in the future avoid issues like this.