Admin bar disappeared

Hi, I have just noticed that my admin bar has disappeared from my main site, though it is there in the dashboard and on all other sites in my multi site install. I thought it was because I have just installed market press, but this doesn't seem to be the problem, the other thing I think it maybe is that I upgraded buddypress yesterday. Do you think this could be it? How could I go about fixing it?

Thank you for your help.