We are currently experiencing a flood of requests from bots scraping the forums. Unfortunately it has gotten to the point where it is negatively impacting the site. As a result the forums may be slow and you may periodically experiance an error message.
We are aware of the problem and apologize for the inconvenience.
In other words, you are either invisible (and as that user mentioned, check the "Edit Options" in User CP") or you may have been inactive in the forums so when you checked again, it appears as though you are offline.
Answered and closed by the Mews. (If you want to re-open it, just PM me. =p)