Q: Guest User can login to my main account without password

Hello. I got two accounts in my laptop, one is my main account and the other is guest account. My friend restarted the laptop while using the Guest account. After it restarted, it automatically login to my main account without any password. I do not want people to easily login to my account without type in any password. I filmed a video as well. I can email it if needed. Is there any way to fix this? Any help would appreciated. Thank you.