Dell Latitude E6540 can’t start windows

Perform a hard reset
1. Power off your Dell PC
2. Disconnect the AC adapter or power cord, and remove the battery (for Dell laptop PCs).
3. Disconnect all external devices or peripherals like USB drives, printer cables, and media cards (SD/xD).
4. Press and hold the power button for 15-20 seconds to drain residual power.
5. Connect the AC adapter or power cord and the battery (for Dell laptop PCs).

If your Dell PC still does not power on or boot into the operating system, follow the appropriate guide below based on your Dell PC's symptoms. You can also follow the instructions below to run a hardware diagnostic check on your Dell PC.

Run hardware diagnostics
When your Dell PC does not seem to be working correctly, diagnostics are available to help you identify and resolve the issue. Dell has built a suite of self-diagnostic tests into Dell laptops, desktops, servers, and Windows-based tablets. These are called the Dell Enhanced Pre-Boot System Assessment (ePSA) or Dell Pre-Boot System Assessment (PSA) Diagnostics.
When your Dell PC encounters a problem, it is recommended that you first try to run a diagnostic test. Any error codes that these tests provide can go a long way toward troubleshooting your issue.

Tried it, still doesn’t work. 
the problem is that the laptop turns off immediately. Is there anyway to sustain it on? And run the diagnostic settings ?

Try powering up on AC only - without the battery.  If the system will POST, the battery is bad; replace it with a new one.

If not,

If the system has more than one memory module, try one module at a time in each socket.  If one module fails to work in both sockets, it's bad;  replace it.

If both modules work in one socket but not the other, the socket is bad;  replace the system board.

If the system still won't power up successfully, the mainboard is bad.