Hey, sorry to hear you're dealing with this. iOS updates can be pretty finicky sometimes. I've run into similar stuff before and here's what usually helps:
1. Check your microphone permissions. Go to Settings > Privacy & Security > Microphone and make sure Alexa actually has access. Updates sometimes flip those settings off without telling you.
2. Try offloading the app. Settings > General > iPhone Storage > find Alexa > Offload App. This clears the cache but keeps your data. Then just tap it to reinstall.
3. If you're using Siri, there might be a conflict between Siri and Alexa fighting over voice input. Try turning Siri off temporarily and see if Alexa starts responding again.
4. Sign out of the Alexa app and sign back in. Sometimes the account session just gets weird after an update.
5. Make sure the Alexa app itself is updated. Amazon might have pushed a fix already for iOS 26.4.1.
6. Last resort, reset your network settings. Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings. Just know this clears your saved Wi-Fi passwords too.
Hope one of these does the trick!