HassOS is becoming unavailable

My HassOS is becoming unavailable each few days and after a reboot is coming back to work.
My setup is a Raspberry PI4 4GB with a SD 64GB (Sandisk Extreme Class 10 U3 A2) SD running 3 months and entire setup completing one year.
I was reviewing the statistics and there is no CPU or RAM overload and temperature at 55°C as maximum.
My question is: What logs do I have to review in order to find the root cause of the issue??
Thank you in advance.