One possibility is that the systems rootfs has been corrupted, this happened to me once with unsafe powerdown on a raspi.
If you have another sd-card, the easiest would be to install a fresh image on it and check if it boots correctly. If not, make an image of your current sd-card, format and reinstall.
Hopefully you've got backups, as there's no guarantee to recover your files if the data has been corrupted.