FreeBSD 10.2 can't load kernel


I had to reboot my server running 10.2 but it didn't boot. After detection of all my disks it only shows:

"FreeBSD/x86 ZFS enabled bootstrap loader, Revision 1.1
can't load kernel"

lsdev shows all my disks and my zfs pools but I can't access any files on / neither loader.conf

I am totally inexperienced concerning kernel problems.

I would appreciate any help :)

Edit: Maybe this is important - The boot disk is an USB external HDD

Edit2: if i enter boot -s or any boot command i get "no bootable kernel"

Edit: I can enter the bootloader by pressing ESC. if i enter "?" i can see the content of (probably) my bootdisk. What can i do to investigate further?
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