I tried to change to chinese language twice and it didn't work. I tried the Spanish and here is how it worked for me:
1. Switch to Spanish (Setting, General, Languge and Region)
2. Search "ok" on text (I Used other word but it turned out zero search found...which was worse then when it could at least search for past two days originally. But I think if i have used "ok"', it might have found some text....cause that was what happened after I switch back to English.)
3. Power off
4. Power on
5. Change back to English
6 search "ok" and then " Thanks" which seem to help the phone refresh it searching function.
7. After tried out those two more common words in most text, the rest search seems to work afterwards.
Hope it works for you! I actually saw this solution on the other post and after my text search worked, I could not find the post to thank the person who took the time to post. Thanks to the kind hearts out there making time to share and help others!