Hi
As you've upgraded has the salesperson performed the swapmysim procedure to move your mobile number and tariff from your old micro sim to the new nano sim.
Has the service stopped on the microsim.
It takes up to 24 hours approx to fully settle sometimes and turning the phone off and on helps reset the connection.
If you ring your own mobile number does the HTC One m8 ring ? If not perform the sim swap yourself here
http://swapmysim.o2.co.ukRing O2 customer Services to have your account checked if the sim swap has been actioned, and us over 24 hours ago.
http://www.o2.co.uk/contactusOccasionally turning the phone off , clean the SIM card and battery pull (phone model dependant) and turn the phone back on clears lots of glitches.do you have MOBILE DATA set to on in the phones settings.
Is their any data connection icon in the phones status bar such as 4g, 3G, etc ?
Check your service status via your full postcode here
http://status.o2.co.ukI assume you have data allowance left to use !
Try texting ACTIVE to 2020, save the returned settings and re boot the phone.
A help link
http://community.o2.co.uk/t5/Android-Devices-Samsung-HTC-Sony/Connecting-Android-Phone-to-Data-Servi...Apn settings are here
http://www.geeksquad.co.uk/articles/o2-apn-settings