General troubleshooting

Try running the most basic Zephyr samples like blinky or hello_world.
Some boards are not supported by MCUboot (and thus have no support for OTA updates). For those boards try using the samples/without_ota project.
Some boards use the CDC_ACM virtual port for debug output. If this doesn't work correctly, try switching zephyr,console to a hardware UART. You'll need an USB to TTL Serial cable or adapter to access the device console.
If your board is unresponsive or misbehaving, it might be damaged. Please try the same procedure with a fresh new board.

Cannot flash the board

If your board exposes multiple USB ports, try each of them. Sometimes, connectors labeled as native are used for flashing.
Check if your PC recognizes your board and the relevant drivers are loaded.
Try rebooting your PC completely (by powering it off and on)

Zephyr has an MCUboot example, that should work out of the box. Try running it and see if your board boots correctly.
When changing the partitions layout, calculate the offsets carefully. We've noticed that DTS compiler doesn't check partitions for overlapping, etc.
If the OTA process fails, try lowering the UART baud rate (i.e. set CONFIG_BLYNK_NCP_BAUD to 115200 or less).