You will not be able to migrate files using the Mass File Migrate app if you don't have any credits left.
For UI migration, you will see the following message on the screen:
"You have exceeded the procured credit limit. To purchase additional credit limit, please contact Satrang Technologies."
For batch migration, the batch will abort, and you will receive an email with the same message:
"You have exceeded the procured credit limit. To purchase additional credit limit, please contact Satrang Technologies."