Can't upload the sketch. not in sync: resp=0x94

I'm using the Arduino Duemilanove with Atmega328 connected to my Mac-mini via USB cable.

It was working fine before.

At one point, Arduino program seemed like it lost its connection. The port name was disappeared from the list. This happened before. And it was okay after I unplugged and plugged the USB cable. So.. I did the same thing. I unplugged and plugged it again. I could see the port name on the list again, but, I noticed Tx and Rx LED both are on constantly.(This never happened before). Since then I can't upload the sketch. Error message is as below avrdude: stk500_getsync(): not in sync: resp=0x94 avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x94

I tried checking Tools>Board, Tools>Serial Port, Unplug and plug. Pressing reset button before uploading, Restart my computer.

I don't know what else I should try.

Have any idea about this?

Can anyone help me to try something else?

(deleted)

Loopback test worked out.

Tx,Rx LED are still on for some reason. But ,at least, I can upload the sketch now.

Thank you.