Try multiple locomotives before assuming you have a system problem!
Did the loco lose its programming? See if it will respond to loco address 3. NOT 0003, or 003 or 03.
Some DCC locos have startup / shut down procedures! On some BLI / QSI decoders and others you can accidentally "Shut down" the loco by double tapping F9. This leaves the sound going but in effect puts on the parking brake! A double tap on F6 or F3 will "start" the loco again.
Does the display on your cab say CON? That means CONsist! Did you add the loco to a consist by mistake? You might not realize this even happened. Browse the consists in your command station memory to find out. Kill / Delete the consist.
Go here for consist help: https://ncedcc.zendesk.com/hc/en-us/articles/205929955-Consist-Troubleshooting
View / read back the value of CV19 for that loco in programming track mode. How to read or change a CV. Do you see something in CV19 other than 0? If yes, the locomotive thinks it is part of a consist! Delete the consist from your system with all the locos still on the track per the consisting instructions for your particular DCC system. To manually clear the consist id from a loco, set CV19 to a value of 0. Reboot your system, it should then respond to the original address.
Is it new out of the box? Did you select loco 3 then press enter? Not 0003 or 003 or 03. must be 3. Does it need to be "started"? Try pressing 3 or 6 then wait a few seconds. Did someone already set it up for you? try the locomotive number on the actual loco.
Some locos have a hidden switch somewhere on the chassis to power on loco. Check your manual! It can also be a magnetic reed switch that is energized with a "magic wand"
If no results yet then the next step is a factory reset of the LOCOMOTIVE. Take all the other locos off the track. Set CV8 to 8.
This will get it back to the factory address 3 . after you verify it will run on 3, then use OPS mode programming to change the address from 3 to what you want.
FYI, this is a very common issue with BLI locos. So might want to practice this.
Take all the other locos off the track. Set CV8 to 8.
after you get it running on 3. To change the address