My new(ish) Dell U271 took a bit of a knock and it now suddenly doesn't recognise the Macbook plugged into it via DisplayPort - this had been working great for several months up until now.

When I "Scan Sources", it doesn't find anything on DisplayPort. However, one suspicious behaviour leads me to think all might not be lost. When the Mac is woken from sleep, this used to kick the monitor in (as an monitor would) and while it no longer does that, the light on the monitor does go from standby orange to blue briefly, then the monitor says it's going into "power save" mode. It seems like it gets some kinda signal.

Any ideas on how I can diagnose the problem? I'm trying to find another mac but no one near me has one ...