It appears that my existence right is being denied on Steam,

It had been working fine but now it says upon startup: Unable to connect to the Steam network 'Offline Mode' is unavailable because there is no Steam login information stored on this computer.
You will not be able to use Steam until you can connect to the Steam network again.

I've been using the same PC all the time and only now has this error started appearing and I'm sure my login still exists as well as my account page.

Would anyone be kind to help me out here?