I think I can help you with this problem
You said that you have already cycled the power to the Labelwriter 400 Turbo, right?
Are you using a Windows computer or Apple?
Ok, do you have the Dymo software opened up?
Not at this point as I am on email. Do you want me to power up Labelwriter?
Ok...Ope the the Dymo Label software. Click File, select Change Printer, verify you are printing to the correct LabelWriter, and print a label.
If you successfully printed a label and have a LabelWriter 400 or LabelWriter 400 Turbo, you must update the firmware in the LabelWriter.
Now working, Thanks.