The Kernel menu shown in the following screenshot allows you to perform various operations on the notebook kernel:

Some important functions include the following:

• Interrupt Kernel: Interrupts the underlying kernel when it appears to be bogged down in a loop
• Restart Kernel: Restarts the kernel if your notebook is not responding due to a function you had invoked in the kernel
• Shutdown Kernel: Shuts down individual notebook kernels 
• Shutdown All Kernels: Shuts down all kernels running on all notebooks
• Change Kernel: Changes your notebook's current kernel to another