Dont load any sketch

Yesterday I was playing with my arduino mega 2560, I was checking the serial communication, today when i connect the arduino to the pc, the pin 13 led is blinking and when I try to load any sketch I receive this error

avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer

I try the loop-back test and everything was alright...
And i reinstall the firmware but it still blinking

mamk90:
And i reinstall the firmware but it still blinking

What firmware?

Arduino-usbserial-mega.hex with atmel flip 3.4.1

If the board appears as a serial port on your PC, then there probably wasn't a reason to reflash the usb to serial chip.

You might need to reflash the actual ATmega2560's bootloader.

How I reflash it???