Problem with uploading

My board has been working perfectly, but suddenly I'm getting this error message when trying to upload:

Arduino: 1.6.13 (Windows 10), Board: "Arduino/Genuino Uno"

Sketch uses 4,036 bytes (12%) of program storage space. Maximum is 32,256 bytes.
Global variables use 337 bytes (16%) of dynamic memory, leaving 1,711 bytes for local variables. Maximum is 2,048 bytes.
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 3 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 4 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 5 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 7 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 8 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 9 of 10: not in sync: resp=0x36
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x36
Problem uploading to board.  See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

I also noticed that the reset button is not working... Anything in my hardware/software hasn't changed, so is my board broken? What could I try to do?

Edit: ON -led and pin 13 -led are glowing when the board is connected via USB, but RX and TX aren't even when I'm trying to upload a program

Finland:
My board has been working perfectly, but suddenly I’m getting this error message when trying to upload:

Arduino: 1.6.13 (Windows 10), Board: "Arduino/Genuino Uno"

avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x36

This is a pretty generic error. avrdude is unable to communicate with the bootloader on the 328p. It could be because the bootloader has been damaged, the USB to Serial Chip has been damaged, or the Windows serial driver is no longer working. Among other reasons.

Finland:
Edit: ON -led and pin 13 -led are glowing when the board is connected via USB, but RX and TX aren’t even when I’m trying to upload a program

Back up. Is the board appearing in Device Manager when you connect it?

Finland:
I also noticed that the reset button is not working… Anything in my hardware/software hasn’t changed, so is my board broken? What could I try to do?

What is it you think the RESET button does? Or what are you expecting to happen when you press it?

Is the board appearing in Device Manager when you connect it?

Yes, it is appearing like before.

What is it you think the RESET button does? Or what are you expecting to happen when you press it?

Before all leds have started blinking for a moment when RESET button was pressed. Now nothing happens.

Follow the instructions on the top of this forum for loopback testing. This will verify the serial connection is working. (I suspect it is.)

After that, either the atmega328 has been damaged or the bootloader has been corrupted. You'd need to attempt re-programming the bootloader or just replacing the atmega328.