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You said this is occuring on your iPhone 6, but you are posting this in the desktop section.
If the problem is occuring on desktop:
The ability to search a playlist will be coming back in a future update. (currently no ETA)
If the problem is occuring on your iPhone:
Try reinstalling the Spotify app.
If all of your music is gone,
I suggest you contact support so they can help you https://www.spotify.com/about-us/contact/contact-spotify-support/
If you get an automated reply email telling you to check the help section or the community, you need to reply back to it, even if it's from a no-reply address.
Support usually replies within 24-48 hours.
You can also tweet to @SpotifyCares on Twitter for help.