Have you replaced the switching power supply?
Is there any component that burn up on the PLC board? A bad power supply that cause a large ground potential between the PLC and the PC often burn up a protection resistor next to the RS232 port. You can try to replace the resistors.
Is the PLC program still running? When you power up the PLC does the 3 status LEDs turn ON for 1 second and then 2 or all 3 (with MX-RTC installed) will go off? This will tell you if the PLC is still alive.
If you turn ON the dip switch #4 to the PLC, does the PAUSE light lights up? If so, the CPU is still running properly.
If all else failed, please email to support@tri-plc.com with your purchase information and request for RMA # and service center to ship your product for repair.