Can't use my site! Restricted access error!!

My site has been live for 18 months on 4.7

In the last week, something has happened to the login functionality:

I'm an administrator of my site.
I login as per usual.
If I click on any menu option that requires me to be logged in, I get:

Apologies
Restricted access

Sorry, only registered members permission to access this page.
login to view this page or register for your free account.

If I login using logintoboggan (which redirects me to the page I was trying to access), then I'm back in at the correct page, but unable to make modifications (since I can't save).

The last time I used my site was about 2 weeks ago. I have not installed any new modules for 6 months (or more). I have not changed any code. The only thing I have done is create content, move content around, delete old things, uploaded some images, create some new categories etc. Everything worked fine last I looked.

In the last 3 days, users (not admins) reported this problem to me, and I can't figure it out.

Anyone have any idea what could be causing this or how I can even troubleshoot it (since I can barely use my site)

Thanks for any help.

Cheers,
Tys.