Sketch not uploading; reset not working...

Hello!
I know this issue is super generic, and that many others have asked about it before, but I've tried everything listed so far and nothing seems to help.

I have an arduino board (a Sparkfun Redboard, AKA UNO) that's been working great for about 3 months.

However, in a recent bit of experimentation, I connected the board's reset to ground (to stop it from running) so I could use the board's UART->USB converter without any interference. I've done this before with no issues, and it worked great this time.

However, now that the wire has been removed, the board does not reset (lights do not blink like they normally would upon reset). The power LED is lit, it is recognized as a COM port, and the little loopback (RX connected to TX) trick works fine. But, when I try to upload a sketch, I get

avrdude: Version 6.3, compiled on Jan 17 2017 at 12:00:53
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch

         System wide configuration file is "D:\Shared\Program Files\Arduino\hardware\tools\avr/etc/avrdude.conf"

         Using Port                    : COM4
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x8a
[....]
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x8a

avrdude done.  Thank you.

Problem uploading to board.  See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

The "resp=" is different every time I unplug and re-connect the board. This error occurs even when just uploading the "barebones" sketch.

System: Windows 10, Arduino 1.8.2

Any help would be appreciated!

Using Port                    : COM4

Do you have the correct serial port selected?

Yes. As I said, the common "loopback" test works fine, so I'm sure that's the right port.

Fuzzyzilla:
As I said, the common "loopback" test works fine...

Which can also occur by coincidence if your computer happens to have a modem or other serial device. Hence my question.

When your board is powered, does the led on pin 13 blink quickly three times?