I've gotten this twice tonight. I log in and everything stays as it was when i loaded in. I can interact with objects on the ground, move stuff in my inventory, use my hotkeys to select items (but if I select a weapon I instantly put it back again), and even use them, but none of this registers on my screen (I had a friend with me who verified my actions). I can see myself move, but noone else (players or zombies) move, and all items (both on ground and in backpack) stay where they were when I logged in. New items don't show at all.

If I relog, it updates, but any new changes are lost. I tried joining several different servers, but had the same bug on all of them. I finally solved it by letting my friend headshot me. I didn't die on my screen and could still run around, but when I relogged I was back at the beach and could play as normal again. The second time it spontaneously solved itself after several server changes and a few minutes of running around looking for some way to die.