Christian, that RB1200 has some age... leaving a device for so long without upgrading rises a lot the chances of problems, moreso if the device (hardware) is old.
How Old is it? ten years?
Looks to me like a damaged main booter.
1.- Connect a serial console
to it, so that you can reach the booter prompt. (Cisco "baby-blue" serial cables work).
2.- Set it to boot
from backup booter. (hit b)
From this step onwards, you should be able to access it via winbox.
You can also monitor it via serial, and force it to connect to netinstall: Use booter settings to set it to ether-boot. RB1200 should be connected to netinstall server via ether10
FYI, 6.42.7 has been released:
*) upgrade - fixed RouterOS upgrade process from RouterOS v5;