If the PLC is frozen (hangs) i.e. one cannot even connect PLC from PC then it is a crash.

In such case one has to restart the PLC and take crash log file.

Caution:
After restarting more than one time crash log file will get lost.

To check the validity, open crash.bin file in notepad. If GO is in the beginning and END is in the middle then it is a valid crash log file.

See sample crash log file (Sample_Crash_Log.txt) attached for reference.