Flashing the firmware is a sensitive process. When doing so, you need to make sure that the instructions found here are followed to the T.
If you weren't able to flash the firmware correctly, it may say "No bootloader found".
"No bootloader found" means that the keyboard is not getting into bootloader mode at the correct time.
This can be caused by pressing this key either too soon or too late during the flashing.
For an alternative method of flashing, please save your layers and reset the Neuron using the instructions found here: How to Reset the Neuron