Yesterday I could find a user via the API:
https://api.instagram.com/v1/users/search/?q=...&access_token=...
But now everything returns Meta-code-200 and empty data.
What happened?
If your app is in sandbox mode then you can get information about only sandbox invited users.
If you app is in live mode then make sure public_content permission is Approved.
Please also check scope you are using. Make sure in scope you are sending public_content permission.