Ah - what a coincidence! My GR started having the same problem a week ago while travelling and I you beat me to posting about it here

It was a quite cold night and my battery was running out - so I was turning the camera off and on again to save on the battery - and on some occassions it would just be dark screen, I thought might be the cold weather causing it... but it was not humid or anything.
So it is the same problem - looks like the diaphragm leafs do not move away on initial startup, but it is inconsistent - sometimes on startup it does open. So I get a dark screen (since sensor is covered and gets no light). It is inconsistent and when it happens the options are either a) taking a photo (so leafs are forced to move) or b) turning it off and on again (as they say in IT Crowd

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I've contacted the seller via Amazon and they've told me Ricoh GR provide 2 year manufacturer warranty. Luckily I bought it less than 2 years ago (Aug 2013), so will be calling the service centre for more details later today. Hopefully they can repair it quickly...
Here is the URL with more details:
http://www.ricoh-imaging.co.uk/en/service-centre.html