(Joomla 3.7.5, MySQL 5.6.32)

I upgraded K2 to 2.8.0 yesterday as it the admin page was prompting me to do so. Previous version was 2.6.7. I made the mistake of doing it in a hurry, and foolishly didn't take a full site backup immediately beforehand :-(

The upgrade process completed quickly, but gave the following error messages:

Warning
JFolder: :delete: Path is not a folder. Path: /home1/handsom1/public_html/components/com_k2/js
JFolder: :delete: Path is not a folder. Path: /home1/handsom1/public_html/components/com_k2/sef_ext
Duplicate column name 'ip' SQL=ALTER TABLE `#__k2_users` ADD `ip` VARCHAR( 15 ) NOT NULL , ADD `hostname` VARCHAR( 255 ) NOT NULL , ADD `notes` TEXT NOT NULL
Error
Error installing component

After this, accessing the web site simply returns a blank page (seems to be giving a Server Error 500). The admin login is available, but after authentication just get blank screen again. I tried reverting back to the daily site & DB backup, but this made no difference to the results. I have subsequently reverted back even further, but to no avail.. I have even tried reinstalling the original version of K2, but get a similar SQL error.

On further investigation, it appears that some of the admin pages are available if I enter the full URL directly, such as the list of K2 items, but trying to open an item just gives blank page again. I am unable to use the 'Manage Extensions' dashboard, although the 'Install Extensions' page is available.

What might be broken here, and why is the file restore not fixing it? I have cleared the caches wherever I can.

As I can't access the Global Properties, I cannot switch debug on. How else can I find out what is failing here?

Any help would be much appreciated to get us back on line :-)

Since you dont have a current backup you should probably do a mysql dump of the site just in case of the worst before making other changes.
First try to edit your configuration.php file and turn error reporting to ‘E_ALL | E_STRICT’ and hopefully you may see an error instead of a blank page that may lead you to know what going on.
Later you may need to:
I would unzip the k2 package file and ftp the entire com_k2 directories on top of your exisiting ones and see if that helps
Since it Broke Joomla, you may have to do the same thing with your current version of Joomla as well
but try to find out whats going on first

Thanks for the reply William.

I managed to get JustHost to restore the site (the backups I specifically pay for do not appear to be useable - that's another matter entirely!) and we're back in business now. However, I haven't retried the upgrade to K2 2.8.0 to see if it works this time. I'll give it a week or so until I have more time, and I will take an XCloner backup before I start.

Akeeba Backup can also help you (for backups).