Cut the power from the board. After 10 sec. repower. Try to upload SaraSerialPasstrough from the MKRNB examples.
Type ATI9 in the serial monitor. This should show up the firmware.
If it's under L0.0.00.00.05.12,A.02.19
Do a upgrade as described in:
The MKR modem is a sensitive diva. After some hard powerlosses sometime she becomes unresponsive..