During the upgrade, SUM tool creates\reads below files.
Due to abnormal crashing of SUM tool, Information in these files become inconsistent.
So when SUM tool is started again, it does not read the proper information from these files.
Exit from SUM upgrade GUI (If it is still running) and ensure that no SL controller or SDT processes are running at OS level.
Take a backup of \SUM\sdt\data\.sdt_storage, \SUM\sdt\data\.sdt_keystore to different directory.
Delete files \SUM\sdt\data\.sdt_storage and \SUM\sdt\data\.sdt_keystore.
Start SL controller.
Launch SUM upgrade GUI. It will start from the point of last crash.
tags: Blank SUM Upgrade GUI, SUM gui not started, SL-Controller, STARTUP, \SUM\sdt\data\.sdt_storage, \SUM\sdt\data\.sdt_keystore, Software Update Manager hangs with blank GUI, sap note 1792305 ,