External monitor (Xerox) not displaying on MBA

I have a Xerox XA7-19i external flat screen monitor connected to my MBA. At first it worked fine, but when the MBA went into sleep mode, the external monitor would not wake up. Instead, it showed a 'No signal' message. I tried retracing my steps, and managed to get it working again, but only after I unplugged the power cable of the external monitor. The problem persists, however, and I can't seem to get it to work again. I have tried different resolutions, detect displays, turning off mirroring etc. with no success.