I tried to setup iSCSI target and defined
root partition as target (that was wrong I know).
I didn't format the iSCSI device; is it possible to get back the data from this partition? System does not boot at the moment.
In theory you could establish a root filesystem in an iSCSI target but you would need to do a good number of things to make that possible. It seems like you were using the blockio method (raw filesystem). I suggest you look-up the fileio method as it will be a little safer for you relative to your experience (no offense intended).
To get the data back from a raw filesystem (used by iSCSI) you would need to move the disk to a working system, establish an iSCSI target for that partition and then connect to it from any initiator.