L led always on and not uploading

I don't know what is happening, but i cant do anything. I'm was playing with an arduino nano (Atmega328p) when it happened. I'm was uploadig a sketch, and it seems like it don't upload at all, or the usb cable discconect for a moment and then connected again. The only things i know is that the L led (from i know, is like an advise signal for the bootloader)is always on until 20 or 25 seconds, and tthen turn off, sometime, it has a fast blink (when i say fast i say faster than normal) and then it stay on. I cant upload sketches, i get every single time an error:
avrdude: stk500_getsync() attempt (1 to 10) of 10: not in sync resp=0x00, 0x84, 0x31, 0x77, 0x79, 0xb7, 0xea, etc, etc. When i try to upload a sketch, the L led turns on again (think for the reset precces for uploading) and the RX led blink 3 to 4 times and then nothing. i try reinstalling the FTDI drivers, pressing the reset button until it says uploading, uninstalling the IDE, Using 328p (old bootloader), changing cable, etc. Nothing gives me a good ressult. the board is now disconnected from anything ( or at least that is that i know), the RX and TX pins are disconnected, I try echoing, and it worked, so i dont know what is happening. please, somewone help me, I need help.

Thanks

Sorry for my bad english

Atmega328p.zip (1.86 MB)

Since you already tried all the things I recommend in the beginning, I would say that the next step is to try burning the bootloader on the Nano. To do this, you will need an ISP programmer. If you don't own one, you can use a spare Arduino board as an "Arduino as ISP":
https://www.arduino.cc/en/tutorial/arduinoISP