is your n900 booting?
if yes you could trybackup maenu.
it makes a flashable image of your rootfs and optfs.
but i thing it doesnt saves your media files.
use the search to find out more about that

Tried from the PC:
And also:
Any idea to backup some data?

It doesn't pass the "running dots" screen, and doesn't reach desktop.

thats really bad dude.
so its your decision what you do.
but if i was you.
i would flash the whole phone and learn something about it.
i make every 2 months a full backup.
i drop the whole mydocs folder on my windows and make with backupmenu a whole image of rootfs and opt fs.
and i make backups with the integrated app to not lose any contact.....

Thanks for the help. Your last message is the best lesson for me: I've taken the decision and wiped the whole phone flashing the eMMC.

Now works freshly as the first day, and clean as the intergalactic space.

Backups!

so im happy that it worked fot you
and never forget to backup.
try backupmenu its really nice.

regards