External monitor suddenly stopped working at 2440x1440 on OS Lion 10.7.3

Hi,
I have a 27" iMac and I'm using Samsung SyncMaster SA850 as a second monitor (connected via a displayPort cable with a mini DisplayPort adapter).
Everything worked fine until yesterday - the Samsung screen turned black and now it only works with max resolution 1280 x 1024 (native is 2440x1440). I think I upgraded Lion 10.7.2 to 10.7.3 (I don't remember for sure though).
When I connect the monitor to an iMac with 10.7.2 everything works fine. And another iMac with 10.7.3 has the same problem as me.
Any ideas what could be wrong, please?
Thanks!
Tomas

It's very temperamental i find.   I have just moved house and when i reconnected everyting i couldn't get the monitor to work properly.   I played around for a while and eventually changed my input from DVI1 to DVI2, then played with the input button at the bototm of the screen (the one just on left of the power button) and somehow it just seemed to click into place...  Frustrating i realise, but now that it's up and running it looks great and, I daresay, looks much better than the 27" thunderbold display in my opinion... primarily because the non-reflective surface is great, but also because it is so easily height adjustable, a feature which unfortunately is non existent on the mac monitor.
good luck anyway...

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