A diode near the usb port in my arduino nano fell down...
can anyone explain me how it could be fixed?
That diode is in series with USB power.
Did you draw too much current from any of the pins?
Anything more than a few hundred mA will overheat that USB protection diode to the point that it could fall off. Test the diode and ask someone with SMD soldering experience to solder it on again.
Leo..
You will need tweezers and a soldering iron, and maybe a magnifying glass. The polarity of the diode is relevant !
Wow, i have never done that !
There are many threads/posts here with that diode burned.
Newbies don't always know the difference between an MCU and a power supply.
Leo..
He's lucky if fell off and even luckier if it fell off before burning out.
I've done this when measuring voltages but neglecting to notice that my multimeter was previously used on a 10A range with the leads left accordingly. It takes only a second or so to destroy the diode.
Hi,
I believe that is not a diode, my Nano has a B2 , a Schottky barrier diode in that position.
@tejas_lives_codes Nano has a 1S4, which I believe is a surface mount slow blow fuse.
1S4 in the image does not have a polarity indicator bar.
Tom... 
does that mean i can connect any way i want?
Hi,
Yes.
If you have a DMM, please test the component's continuity in both directions to check it's function.
Tom...
is it a good idea to use an adhesive
Is this how SMD fuses look like?
Is there any fuse on Arduino Nano???
What if the marking is only "S4" (very common Schottky) and the "1" is the polarity making?
Google tells me that 1S4 is a Schottky diode (1Amp/40volt).
The line that indicates the cathode is sometimes barely visible.
Leo..
That was also my suspicion. (i couldn't prove it with google though) It happens a lot on SMT diodes actually. (that the writing fades) More than likely the Cathode is on the left when you are reading the writing, I normally test with a resistor & LED though to make sure. But that still leaves to figure out where the cathode should go, I guess the Anode goes to the 5v of the USB hub right ? that can found with a multimeter.
No. there certainly is not. The Polyfuse is on the UNO.
So it isn't a fuse.
But maybe it is now. Need to test it before going further. 
Actually, the fact that it overheated indicates it did not "fail open", but it probably is no longer a diode. Rather a "dear departed diode". 
Modern Surface mounted technology !
What does that mean.
I had the biggest problems (over many decades) with through-hole.
You also would need a wheelbarrow to cart your cellphone around if it had through-hole parts.
Surface mount is the best thing since sliced bread.
The solder of that diode probably didn't flow properly during manufacturing.
Surface tension of the solder usually keeps parts firmly in place.
Leo..
i was talking about @ZX80 solution. SMT is great, i can make stuff much smaller now. Though diodes can be a pain.
And it may not have been all that much solder. A little more is waaayyy stronger.
Thanks all for your suggestion guys!
I fixed the diode with an adhesive but not disturbing the pins, so if anyone cannot solder their diode or whatever component use an adhesive but without making the adhesive touch the pins of your broken component so that the current flows 
Hi,
Did/have you tested it with a DMM in diode test mode?
Thanks... Tom...