The "loading bar" seeming to hang is an issue with the current sandbox and will be fixed in a very near-future release. Underneath the covers, though, services are started/starting - even though that screen appears for a while.
Ty opening up a browser and navigating to: https://localhost:8080 (or SSH via the browser at localhost:4200). If these services load, then the sandbox is up and good to go - that loading screen is just hanging and should change a few minutes later.
Newer releases on the HDP sandbox won't have this issue, and the current HDF sandbox is free of this already.