About 14 days ago, RPi Kernel Version 4.4.38 was published. However, something went very wrong somewhere: Raspbery Pi Models 1 and 2 do not boot anymore. As a quickfix I would recommend to download the 4.4.37 Kernel from the Github Repo (https://github.com/raspberrypi/firmware/) and replace the boot Partition on your RPi 1 or 2 SDCard with the /boot path from the 4.4.37 ZIP file - and it should boot again.
If you're RPi is still working - do not update your kernel until this problem is solved! (Issue on Github).
EDIT: Reason for the issue was mostly the open
device_tree=
configuration in the config.txt
Removing this option solved the problem.