why would the overdrive suddenly stop working

Hello, There are multiple potential causes to your concern. There are senors and electrical components that feed information to a processor that will influence shift timing. If data received by the processor is inaccurate, shifting will not occur properly. Alternatively there could be an internal electrical concern with transmission operation and shifting, which can be determined during an electronic diagnostic test using a proper diagnostic tool and shop manual direction. Lastly, an internal hydraulic issue could be present, meaning no matter what the processor is commanding, and no matter what the internal electronic components are triggering, the hydraulic components are not responsive. Testing will illuminate the concern you have.