There are various reasons for SQL services to disconnect from its operations. Typically it is one of the reasons from below,
1. Improper shutdown of the windows system
2. If there is more than one SQL service running for any other third-party applications, there may be a conflict in starting the desired SQL server leading to the stoppage of the services.
3. Windows Updates will stop SQL services until the system is restarted or until SQL service is started again.