Hi and welcome to Just answer.
Initially, remove bobbin and bobbin case then inside of your machine (bobbin area) blow compressed air to clean it. There is a sensor there that possibly block.
On upper thread, try to clean the tension disc using scrap fabric and move it forward and back between the tension disc space. Always thread it with presser foot up. Remove the face plate and blow compressed air.
Put back together then try again. If still, you possibly have faulty sensor that need to check,repair or replace by the authorized Janome dealer.
Hope this information will be helpful.
IMPORTANT: RATING IS REQUIRED!! If you are satisfied to my answer please don't forget to rate,,EXCELLENT OR GOOD rating and BONUS are greatly appreciated...