Hello
I was making some project and I think i wrongly connect somewhere GND and plus.... so in result my UNO stop answering via USB connection.
project is small GSM module... so i connect its power to 5V on Arduino and to UNO GND... module wasnt working, so i throw it to can (it was cheap and a bit damaged)... so in next day i was trying to connect UNO to PC for next project and its not recognizable in PC... it shows that something connected but its unknown. i tried to update drivers but its says wrong drivers. before its was working fine. when connect to USB lights on UNO lights same and if pressing reset button - lights at L flashing - like usual, but PC cant recognize it. whats could be wrong? thanks
If it's an unknown device, and it's a genuine board (or faithful clone - something with an atmega16u2), and it used to work, and you think you screwed up ground/power, you probably trashed the 16u2... The 16u2 seems to usually be the first thing to go if you put something bad onto Vcc (particularly while it's connected to USB) - in contrast, the regulator and LM358 seem to be what goes if you put something bad on Vin/barrel jack.
If it's a clone with an FT232 ftdi chip, is it possible that it's a fake, and your computer updated drivers to a version that doesn't work with the fakes? (google ftdigate for more info).
Do either of those If's make sense with your observations?
The thing to look at on the board is the chip near the USB connector - that's the USB serial interface chip.
Oh - and you say it's unknown device, not a "this device has malfunctioned" message after plugging it in?
its clone, but with CH340G chip... it was working before on CH340G drivers... its like not really listed in device list - i connect it, pretty fast shows that unknown device connected, so i fast press on it twice, then device in list goes nowhere... but in window that I opened said same - unknown device..
not sure what i done to it... here is picture of same device
If it's a CH340 and it's showing as unknown and you can't update the driver (to the CH340 driver, not Arduino driver folder) or it disappears then it's as dead as a dodo. Or at least the CH340 is. Easiest fix, buy a new one 
but how to check is that CH340G? if that something else? and why GND and 5v going throught Ch340G? what else can be?