"Could not find Cortex-M device in the JTAG chain!" (TM4C123g)
This is the suggestion from the Keil website:


  No Cortex-M Device found in JTAG chain No Cortex-M processor-based
  device detected (using JTAG). Device is not connected, not powered, or
  the debug interface is not working. Enable the SWJ switch in the ULINK
  USB-JTAG/SWD Adapter section of the Target Driver Setup - Debug
  dialog.


Check you have power to the device, power cycle the probe, check the probe settings. If the target is crashed, its possible that you'd see this. Try lowering the JTAG frequency. Check the probe has a correct target voltage connection.

The Keil forums might have some more iseas you can try.