There is a "bug" in Joomla update if you try to upgrade from a Joomla version prior to 3.6 straight to Joomla 3.8. This results in your Admin (backend) showing a white screen and there is no way back without some very complex MySQL adjustment which are way above the many mortals who just want to update their Joomla website!
The first time i tried this i ended up with an Admin section white screen with a title tag error saying "error 1054 Unknown column 'client_id' in 'field list"
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