There is an LCD Backlight Panel that illuminates the screen, and yes, they do fail.
Before you start, just check the screen contrast dial inside the top cover, front right, they can be easily knocked and this will also cause the screen to go light or dark.
Janome no longer supplies the LCD Backlight, however, we have an after market replacement at our store, and also provide fitting instructions and a good deal more besides.
If you care to contact me I can send a Diagnostic Routine and the Screen Reset instructions for you to check if there is anything else causing troubles.
Please remember to give details of your dilemma as we do receive a lot of requests.
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Cheers
Martyn