Nikon D3100 mirror jammed

..well, I'd take it back to the store if I were you.

Did you try a hard / factory reset?

That was mentioned in the some of the threads I'd seen, but couldn't work out how to (maybe I should read the manual)

I had a look, some fora say this:

Two of the buttons on the camera have green dots or green labels next to them. Press and hold them for a few seconds and your camera will be reset to factory defaults.

Depends on the type/version you have, some have the green points, some don't.

Two of the buttons on the camera will each have a green dot next to them. Pressing and holding these two buttons will reset the camera to defaults.

Also you could remove the battery. I've had to do this a couple of times to restore a camera to sanity, but judging by the error message you're getting, and the fact that when you press the shutter release the mirror moves a little, I do fear it's in need of major surgery.

this is a common problem for nikons. the fix is quite simple:

step 1: put camera into the bin

step 2: buy a canon

more seriously, the mirror could be jammed mechanically or due to low temperatures etc. normally, i would try to unjam by physically moving the mirror myself by hand and seeing what is wrong (e.g. mechanical problem) or whether there is an electrical problem.

seeing as it is new, you might want to return it and get another camera.

EDIT: i googled it for you and found this:

http://www.flickr.com/groups/d40slr/...7608920360100/

and one fix (looks like mechanical issue with gear):

EDIT2:

some people try to lube it it up to reduce time before it happens again:

Take it here, or send it with a copy of your warranty.

Nikon AG

Im Hanselmaa 10

8132 Egg b. Zürich

043 277 27 00