Unable to start computer after using sudo crontab -e I've set it so that my Mac will forcefully shutdown at 12:00 everyday. But I try to turn it back on, it shuts down again right after.
What command did you put in the crontab file itself, shutdown -h now ?

Boot into single user mode (Command-S) to edit out the offending crontab entry and try the pmset method.

The command sudo pmset repeat shutdown mtwrfsu 12:00:00 (noon) or ...00:00:00 (midnight) will accomplish what you are looking for without using crontab. 

To cancel this, execute the command sudo pmset repeat cancel