my Messenger account looks fine to me, I’m signed-in and ‘available’ – but none of my contacts can see me, to them I appear ‘offline’ and they don’t receive the offline msgs I send them

I have several computer-savvy contacts, they all think I'm a dork ........ my Messenger was working fine up till Tuesday, I haven't changed anything, but now no-one knows I'm on-line ........ I just seem to be totally invisible (they tell me this, very forcibly, by mobile 'phone ) ..... I  haven't changed any settings, as far as I am aware, I am still signed-in (I have tried signing-out-and-in-again, four times ... it makes no difference) ........ HELP!!!!