MySQL Server keeps hitting 100% CPU

My server has 800+ days uptime, but ever since a couple weeks the **MySQL server** keeps hitting **100% CPU** and I have to restart the mysqld service. Where do I start to find out where this issue is coming from? As all the sites on my server keep freezing until I restart. I don't want to keep restarting the service every time. Sometimes it is after a couple minutes, sometimes a couple hours, sometimes a couple days. Nothing changed really. I only once updated the my.cnf file for the default character set and restarted recently, but I don't believe this is linked in any way. **--- Server details ---**
Debian 10 Buster
MySQL Version: 14.14
Average users: 150 - 200 per day
vCPU's: 4
RAM: 4GB
**my.cnf** No slow queries shown so far. [mysqld] collation_server = utf8mb4_unicode_ci character_set_server = utf8mb4 sql-mode="NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION" slow_query_log = 1 slow_query_log_file = /var/log/mysql/slow_queries.log log_queries_not_using_indexes = 'OFF' long_query_time = 5