Lost Keychain and Apps won't open

Hi,

Please help!

I just came home to my imac and there was a keychain error on the screen. Assuming it was nothing I clicked away from it and tried to open an app. But I got an error message saying I couldn't open the app as it was either damaged or incomplete. I was then told that photos has quit unexpectedly. I tried to open safari and got the same 'cannot open as the app is damaged or incomplete' message. I tried other apps but got the same message for all of them.

I restarted my iMac and it told me I had to enter all my passwords for email and iCloud again and most of the app icons have reverted to the OS X app placeholder logo rather than their own icons.

finally my mac was telling my that the updates had been installed.

currently I can't open anything.

any ideas?

Many thanks

Ben