You’ve got a couple of choices on how to do this.
1. Setup a stored procedure and mark it to be run when the SQL Server automatically starts. Setup code within this procedure to use Database Mail to send an email notice out.
2. Setup a SQL Agent job to run each time the SQL Agent starts. Have the job step send out a email using Database Mail when the job runs.
3. Setup your normal monitoring solution to monitor the nodes for a service start event on any of the MSSQL$????? services on the server.