Melzi V2 lost USB connection

repost from the electronics folder as there was no response - hope someone here can help.
Hi all.
During printing the other day I had my Prusa clone jamb to the extent that it tripped the house mains supply.
On powering up again every thing works except there is no communication via USB. My Laptop doesn't respond (no comport recognition). I have changed the cable. Checked the Laptop is OK by using my other machine.
I have control of the printer via the LCD controls and can print from SD.
Its a Melzi V2.0 board running repetier firmware. Any ideas where it is broken? and how to fix?
I can't find any reference to this failure on the web. I think there is only the connector, TTL (is that the protocol ?) chip and a couple of capacitors before it goes into the processor.
If its the chip can I buy a USB to TTL lead and solder it in place of the on board chip?
I checked and the processor powers up when the jumper is set to USB 5V so the 5V is there.

You need to mention which host program you are using

Check
-Com port baud rate
Both the Melzi and the host program must have the
same baud rate in order to communicate

I Asume the hamper for power source is correctly set

Melzi's serial port chip is usually a fake one, so watch what version of driver
You installed onto your PC

Lastly, turn on your printer first and then your PC
Once you lost communication with Melzi, you would need to restart your PC

Hope this helps

Good luck

Hi Bobyni. Thanks for taking the time to reply.
I am pretty certain the issue is on board as the laptop worked OK before and is still OK on another printer. Its not recognising the com port so the baud rate etc is irrelevant.. I am getting the 5V through the USB.
I think there is a TTL converter chip on the board so I was wondering if that's blown, or the serial inputs to the Arduino? I am a novice in that area so hoped someone on here would know how I could check them.
Any one know if the serial inputs are also used by the LCD controls as they work.