Hi @ashborn
Do you have a DMM (digital multimeter) and know how to test electronic circuits?
If so here's the ifixit [guide|9680] guide, that will help.
It shows how to open the power bank so that you can use the DMM to start to test where the problem is.
Tests should include is there a voltage on the battery terminals, is the battery OK, is there voltage being supplied by the solar panel etc?