You may view your 14 digit Amazon Merchant ID by clicking on your name in the upper right corner (after logging in to InventoryLab) and selecting My Account in the drop down:
Scroll down to the Amazon Account Binding section for the information:
You can also Re-Bind your InventoryLab account with Amazon on this page. Click the Rebind button to begin.
You will then be taken through a short binding process with your Seller Central account which, at the end, will put you back into InventoryLab.
Please allow 24 hours for any missing data to sync to your account