Sudden reboots on Open OS 18.02.0

i couldnt find anything helpful, about this topic. someone said 1 or 2 years ago that the button of the slim case is causing sudden reboots, but that is not the case for me.
i updated the system some time ago, no problems. but today it has started to reboot suddenly, i thought before it is the button of the slim case, because it is broken(the part which should hold it in place isnt holding it anymore). but that happened months ago. so i removed the case for now. just now i typed smth on telegram and after i sent it, it rebooted. so it doesnt have to do anything with button.
any ideas how i could fix that? thanks.

Please try the tips in the #rebootsguide and if you still have reboots then continue in the topic linked there to avoid duplicates.