Can I fix an inner tube if the valve has been pushed inside?
Thanks for the photo! This makes it clearer that your valve body didn’t fall into the wheel rim, rather your presta valve internal valve shaft either fell off or fell into the tube body. 



Unlike the photo above, it looks like you have a one piece valve so the core isn’t normally removable (you can’t unscrew the part of the valve pictured above with the words unscrew here) — but if you were to unscrew the thumbscrew too far the inner valve shaft itself  could fall into the inside of the tube. 

You’ll need a new thumbscrew at minimum but it might be very hard to get the thin internal valve shaft back up into the valve body properly. Unlike a tube with a replaceable core, I don’t think you can replace the core if you can’t rethread the valve shaft  from inside. 

In many ways, it’ll be easier to just replace the whole inner. They are cheap for a reason.