I've basically lost a screw on my glasses with the lens popping out of the frame rendering it useless. Is there anywhere I can get it repaired in Zurich? Can I just walk into Fielmann's and get it fixed straight away even though I didn't buy the glasses from them?

I did similar at Visilab in Basel, and they fixed it no problem.

Yes you can.
I've never been charged. Customer service in action, even if you're not their customer.

Any optition will do it for you.

Any optician will do this on the spot for you (and not charge you). I've had a screw replaced in the US by an optician who knew he would never see me again. There seems to be agreement worldwide amongst opticians to do small repairs like replacing screws for anyone who walks in.

Thanks guys!