As we established during yesterday's rollout of the latest Windows 10 preview for phones, there's first an update to Windows Phone 8.1 Update 2 to take place on phones that haven't been running the first build. Unfortunately, after the Update 2 finished installing for many there was no onward route to Windows 10. Now, according to Microsoft's Gabe Aul, things should be fixed and back to normal.
"@GabeAul: If you were stuck on GDR2 and not getting the update to W10 offered, try checking for updates now. We believe it is sorted out."
So there we have it. Go forth and update.