The most likely case here is that your "modem" firmware is corrupt.
IF ON KITKAT (4.4.4): I would suggest reflash of the factory modem firmware package available from here (note this link is the FULL firmware package, download and extract the 2 necessary files).
You must have working fastboot and drivers installed so that the device is recognized by fastboot devices when in the bootloader. Then extract the correct firmware image for your device from the link above to the directory that contains the fastboot executable, and open an elevated command prompt (Right-click and select "Run as administrator" in Windows, or perform a sudo su in Linux prior to starting) and change to your fastboot directory.
Enter:
fastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot flash fsg fsg.mbn
fastboot erase cache
fastboot reboot
This will reflash the device's modem firmware. If this fails you can attempt to flash the full image to the device, instructions are in the linked post above.
IF ON LOLLIPOP (5.0.1): the procedure is the same, but substitute the Lollipop modem firmware from here.
Make absolutely sure you have the correct modems, flashing the incorrect modems could result in "undesirable" effects, although your current condition isn't of much use either.