Ensure APCu functions have been enabled in your PHP install
Design and Skin Customization
Something has changed on your server - if you're not seeing the load values anymore, then it means that the system() function call is no longer working (or being blocked). I have a feeling a server update might have been performed on your server - I would double check with your hosting provider. Nothing has changed in regards to the load levels in the dashboard in a really long time, so if that just stopped working then something on the server changed.