They show on the site, but the admin has no K2 items. Seems to have been an upgrade as the K2 CSS changed on July 15 (which we overrode to get the site back the way it was).

http://imgur.com/a/HclbF

(Last update: March 16th, 2016 - Version 2.7.0)
Web Server Apache/2.4.18 (Unix) OpenSSL/1.0.1e-fips mod_bwlimited/1.4 mod_fcgid/2.3.9
PHP version 5.6.22
MySQL version 5.6.31
GD image library bundled (2.1.0 compatible)
Multibyte string support Enabled
Upload limit 1024M
Memory limit 1024M
Open remote files (allow url fopen) Yes
Check Result

media/k2 Writable
media/k2/attachments Writable
media/k2/categories Writable
media/k2/galleries Writable
media/k2/items Writable
media/k2/users Writable
media/k2/videos Writable
cache Writable

Check Result

mod_k2_comments Installed
mod_k2_content Installed
mod_k2_tools Installed
mod_k2_user Installed
mod_k2_users Installed
mod_k2_quickicons (administrator) Installed
mod_k2_stats (administrator) Installed

Check Result

Finder - K2 Installed - Enabled
Search - K2 Installed - Enabled
System - K2 Installed - Enabled
User - K2 Installed - Enabled

Check Result

AllVideos (plugin) Installed
Simple Image Gallery Pro (plugin) Not Installed

Most likely the table which holds the items (#__k2_items).
You need to repair the database's tables using a tool like phpMyAdmin

I'm having the exact same problem. I have built a website for a major law firm here in London and I cannot work out why this has happened. Any help will be greatly appreciated.

What can I do to get some support? I'm happy to pay for it...

any ideas?

Thank you
A

Step 1:
log out and back in again

Step 2:
migrate to wordpress

@Caroline update to K2's latest version.
@Arkin try repairing your database's tables.