I was reseting my MacBook Pro and it suddenly stopped and its showing apple.com/support -3403f. What I must do now?

"It was running OS X Yosemite before" before you did what, and how did you do it? Are you now running Mountain Lion as shown in your profile? What were you "resetting," and how were you doing it?

What specific MBP model do you have? Can it use full network recovery or just Internet recovery? If the latter, then you can only reset the computer from the Recovery HD as follows. If that failed due to loss of Internet connection, then you need to do it again. I would recommend using Ethernet instead of Wi-Fi. It's not only faster but more reliable. Be sure you computer is connected to its AC charger.

Install or Reinstall OS X from Scratch

Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears.

Erase the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.

2. After DU loads select your startup volume (usually Macintosh HD) from the

left side list. Click on the Erase tab in the DU main window.

3. Set the format type to Mac OS Extended (Journaled.) Optionally, click on

the Security button and set the Zero Data option to one-pass. Click on

the Erase button and wait until the process has completed.

4. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible

because it is three times faster than wireless.

This should install the version of OS X that you had installed.