Mini Display Troubleshooting

Does anyone know how to troubleshoot a mini display port, mine just stopped working,i plugged my display out and when i got home from the office my external display there did not pick up so i thought it was the adapter, got to the office and same problem here now, when i plug the adapter in(without the screen attached) my macbook air's display dims and goes back on- showing me the adapter works and the port works? i tried plugging it into a colleagues screen- same issue, does not pick the screen up?