Process com.android.phone stopped working error

Hi there,
I have HTC one which is indicating that process com.android.phone has unexpectedly stopped working, everything is working okay apart from network bars are missing and I cannot make calls, I have restarted the phone but immediately the phone accepts SIM card password, it flashes the message, I have not rooted nor changed the OS, what could be the cause of this? How do I get my phone working?
