Many iPhone X/8/7/6 users are facing issue after upgrading their iPhone to latest iOS 11.2.6 that Siri has stopped responding to “Hey Siri” and does nothing even after repeated tries. It seems that "Siri Can't understand me anymore" The issue was not with earlier version of iOS 11 but seems to occur frequently with this release. Even the latest iPhone X and iPhone 8 are reported to affected with this upgrade and it seems that mic is not at listening to user at all and thus Siri not starting.
In case you are also facing Siri not working issue in iOS11.2.6 issue please try below solution to fix the issue:
Fix 1: Reset Siri again
Go to, Settings -> Siri&Search -> Tap on the Siri button to turn it off, and turn it ON after seconds. Now hard reset your iPhone. This will restart the siri service. Read here about how to completely reset Siri
Fix 2: Change Siri Voice And Language Settings
This is a workaround which worked for some iPhone users. Change Siri’s voice gender by going to Settings, tap on General and hit Siri. Here, find Siri Voice feature and tap on it to switch the voice gender. Now Change the Siri language as well by going to Settings -> General -> Siri -> Language.
Fix 3: Toggle Dictation Settings
Make sure your iPhone mic are working properly, you can test it by sending some voice message to friends. Once sure about mic settings, check the dictation settings. Go to Settings -> General ->"Enable Dictations". Turn this "OFF" and wait for a few seconds. Finally turn it back on.
Fix 4: Plug it in to a power source it will work
This is surprising but for many usres this solution seems to work Siri on iOS11. Plug in your iPhone to charging or plug it to power bank siri seems to work properly.
Fix 5: Check iPhone Restrictions
Be sure that you have not enabled Restriction option on your iPhone 7 and disabled Siri. You can go to Settings > General > Restrictions > toggle on Siri & Dictation. Then you can test whether Siri is normal to use or not.