Every so often, Amazon's authorization token expires, which takes away InventoryLab's access to your Amazon account.
Re-binding your account to Amazon's MWS should solve your problem! It's easy to do :)
Login to your InventoryLab account
Click your name in the upper right hand corner and select My Account from the drop down menu
In the My Account section of the Settings page, scroll down to the Amazon Account Binding section and click on the blue Rebind button:
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