This is just a public service announcement because a typical N900 user isn't likely to have read enough threads on t.m.o to be aware of the solution to the problem i recently encountered.

Symptoms: The microSD card is suddenly not visible to the N900; the camera app doesn't open when slider is opened.

Cause: The tiny magnet glued to the case underneath the kickstand (it's a ~0.5 cm long black rectangle) came loose and fell out. This magnet is how the N900 senses that the battery cover is on the device...

Fix: find and re-glue magnet or replace with small piece of cuttable rubber magnet material (e.g. printed refrigerator magnets used for promotions).

I've the same problem but looks like the magnet is there, isnt it?
What else could be?

I took some photos

I'm in the same boat as you Ivan. Camera doesn't launch when I open the shutter, and my memory card won't mount. dmesg | grep -i 'mmc0:' shows that the cover is open.

I can't take a picture because the only camera I have to hand is... the N900.

It may be worth heading over to this thread to see what it has to offer.