The cause might be one of the following.

 

1

There might be an abnormality in door's power source.

【When using batteries】
Please reinsert batteries in correct manner or change the batteries.
How to change batteries

【When using AC adapter】
Please reconnect the AC adapter with power outlet. 

【When using controller unit】
Please restart controller unit's power source.

2

The device might not be connected with the power source. 
Please contact the construction company, construction contractor, store which you have purchased the product from or LIXIL repair center.