The default internal monitor is loaded from 0x0000-0x11CD. Make sure your application code is not stepping on the monitor code. In additions to verifing the Com port selection is correct, check the baud rate. In the Kiel tools it should be set to 19200 for the EZ-USB and 38400 for the EZ-USB FX. Try using the external version of the monitor (mon-ext-sio1.hex or mon-ext-sio0.hex).