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"
In my case I found this after upgrading a Joomla 2.5 website for a customer. The first upgrade from 2.5 after all the usual trials and tribulations takes you to Joomla 3.5.1 and from there you should be able to go to the current latest version, which today is 3.8.0. Unfortunately that isnt possible directly even though it is meant to be.
So for now if you have this issue, just follow these simple instructions to go to Joomla 3.6.5 so that you can "one click" update to 3.8 and beyond.
1. Firstly take a backup of where you are now so if it goes wrong you can start again!
2. Download Joomla 3.6.5 from Joomla https://downloads.joomla.org/cms/joomla3/3-6-5/joomla_3-6-5-stable-update_package-zip
3. Download this file postupdate.zip unzip it and upload it to your Administrator root directory so it is at https://www.yourdomain.co.uk/administrator/postupdate.php
4. It is vitaly important that after the next step you dont do anything in your admin section so I suggest you logout properly.
5. Upload the Joomla Update package to your root directory and extract it over your current Joomla installation. This will update the files to 3.6.5 BUT not the database which is why you must not touch the admin section until the next step is completed.
6. go to https://www.yourdomain.co.uk/administrator/postupdate.php and run this file which just means pressing ENTER. You will just get a message saying "You are now running Joomla 3.6.5".
7. IMMEDIATELY REMOVE the postupdate.php file as this would be a security risk to your site.
8. Go to EXTENSIONS - MANAGE - DATABASE and click FIX to update Table structure if it is needed.
9. Each time I have done this I have successfully ended up with a Joomla 3.6.5 site which has properly updated to 3.8.0
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