Admin CP sidebar issues

[jimnyc]

This just started today and I have no reason why. No changes have been made.

When I click on ANY option on the left sidebar, the option opens, but the left sidebar menu completely disappears and only the option I chose appears. In my attached picture you can see what happens. I click on Users/Search for users and the search page appears as the entire page, minus the menu, and this happens with no matter what option I choose.

Any ideas on what could cause this?

[jimnyc]

For further clarification, here's another screenshot with the menu I'm speaking of that disappears, circled in red on the left.