O2

I am currently overseas, I haven't topped up for a while but still got some balance in my account. Have not used phone for sometime and when tried to make a local call it could go through and got this message "outgoing calls barred" could not get in ...

Suddenly my balance is set to zero, I cannot call or text. Iam unable to chat with O2 . My O2 application keep me signing out if I try to use any thing on menu.