Let yourself be upset:
It's totally normal to cry and be upset about receiving an ATAR you consider 'bad'. You are allowed to be upset about it and you should let yourself be upset. The worst thing to do is to bottle it up inside because eventually, you will end up having an even bigger meltdown. If you put in a lot of effort and time into your year 12 subjects and still didn't receive the ATAR you wanted, then you have every right to be frustrated, mad and crushed. Part of the process of receiving a lower ATAR than desired is to let yourself be upset.
However, eventually, you need to get over it. After you have had your time to dwell on your ATAR, wipe away the tears, and try to move on. It's easier said than done but trust me, you can't spend your whole life or even your whole day upset.
Surround yourself with loved ones:
Spend the day with close family members. I know telling your family can be hard but if you put in a lot of effort throughout the year, then they will understand that you did everything in your power to do well in year 12. Your family will support you no matter what and will lift you up in this time of need. Also after you have had your time to cry and be upset, being surrounded by others will help you not think about it. Obviously tell your family your ATAR but afterward talk to your family about other stuff not related to school. Try to get your mind off it and just relax.
I wouldn't recommend talking to friends (who are also in year 12) about your ATAR initially. This may make you feel more upset hearing about them getting into their courses. I'd say try to stay off your phone and in particular, avoid texting friends to hear if they are happy because that may just make you more sad about your own ATAR. Before you celebrate your friends' success, you need to take care of yourself and only ask them when you're ready.