So, I have done my research and tried all of the solves, but I am stumped. Have an Uno (genuine) that I had been using for DCC++ with JMRI. Had been working flawlessly and then all of sudden it all stopped working. Board is correct. Arduino/Genuine Uno. I am using a Mac (Mojave) and the serial USB port is detected fine. However, when I go to the Serial Monitor set at 115200 baud, it no longer gives me the initial read out. This has led to the inability to upload anything onto the Uno. Can't even upload Blink. I have also tried the hold the reset button while uploading. Still get the same response of:
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x00
Problem uploading to board. See Arduino Help Center for suggestions.
BN: Arduino/Genuino Uno