If Signal disconnects from the internet will you lose all your data?

No.

Signal stores all the data available in the platform. 

In case of an internet outage - as soon as the connection is restored all results over that period of time will be displayed on your feed in the Signal platform.

*Please be aware: If you are in the middle of creating a new search or modifying groups/searches - the changes will not be saved in the platform until you click "Save". 

In case of internet outage or other disruption, if you had not yet saved your changes - you will need to start the process again.