Thanks for posting. Apologies that your system is not performing as expected.
Is there any damage on the system? Was the system dropped at any point?
The power button on the palmrest and the power button module itself is located below the palmrest. It is required to open the chassis and disconnect almost all components in order to replace the module.
You could however, remove the keyboard and carefully reseat the power button FFC cable and check if it works. Download the service manual here: dell.com/support
If the power button is functional, then I would suspect the mechanical plastic button on the palmrest might have worn out and hence the issue. In that case, the palmrest assembly needs to be replaced.
If the system is under warranty, please contact me privately. Be sure to include your personal information (name, address, telephone, email) and your computer's service tag number in your message. Thanks.
If there is no warranty, then you could contact our Out of Warranty team to get a quote for a paid service call -http://dell.to/1vnT6CQ