Facetime imessage suddenly stopped working Yosemite 10.10.3

After months of problem free use suddenly both iMessage and Facetime stopped working on my Macbook air, IMessage fails to deliver Facetime calls never get through. I've stopped imagent, checked/updated Flash, signin signed out, restarted - still not working. Apple IDs are fine loggin to email is fine. Just simply stopped working. There have been no updates installed at the right time to possibly have caused this...any suggestions? Surely this is not going to require reinstall the whole OS! Might be helpful if the error message was more than just Mesage failed to deliver - do you want to try again!