I recently got the Bose QC35 II and siri has never worked for me. I hold down the button for 2 seconds, siri makes a beep and then it goes away or it sounds like it's listening for me to talk which I do and then nothing and goes back to normal use.
Thank you for taking the time to post and welcome to the Bose Community.
I apologize for the trouble you're encountering with Siri not working with your headset right now - this certainly is unusual.
What I'd advise doing is performing a reset on your headphones, as well as restarting your mobile phone. Have you been able to try any other mobile devices to see if Siri is responding correctly? What model of the iPhone do you have?
Let me know if the troubleshooting steps fix your issue!