Doorknob only turns one way
If it would previously turn either direction and now does not , this is indicative of either something physically blocking the mechanism inside or needing lubricant. Either way your solution is the same- disassemble and lubricate. Remove the knobs, then remove the latch assembly. Remove any obvious pieces of metal or debris that are present, add a lubricant, work the mechanism, and reassemble. 

https://m.youtube.com/watch?v=WX8NG0275R4