Hello,

The flashing led is letting you know what the issue is look inide this manual expand troubleshooting =>System-diagnostic lights .

1. Turn off your computer.
2. Disconnect the power adapter from your computer.
3. Remove the base cover.
4. Disconnect & remove  the battery.
5. Press and hold the power button for 20 seconds to drain the flea power.
6. Reconnect and install the battery.
7. Install the base cover.
8. Connect the power adapter to your computer.
9. Turn on your computer.

Remember when working inside a laptop you need to take anti ESD precautions.

... and those 9 steps didn't make difference. The laptop didn't turn on

Perform a hard reset.

1. Turn off the computer.
2. Disconnect the AC adapter or power cable from the computer, and remove the battery (for Dell laptops with removable battery).
   NOTE: For Dell laptops that have a nonremovable battery, see the Dell knowledge base article  How to reset Real Time Clock (RTC).
   For Dell laptops with removable batteries, see the  User Guide for your Dell laptop for step-by-step instructions to safely remove the battery.
   NOTE: On select Dell Latitude laptops, try to recover the laptop using the forced ePSA method from a POST or boot failure. To learn more, see the Dell knowledge base article  How to Use Forced ePSA to Recover from POST or Boot Failure.
3. Disconnect all devices such as USB drives, printers, webcams, and media cards (SD/xD).
4. Press and hold the power button for 15 to 20 seconds to drain residual power.
5. Connect the AC adapter or power cable and the battery (for Dell laptops).
6. If your Dell computer can turn on but is unable to boot into Windows, identify the symptoms and follow the appropriate troubleshooting guide.

Thanks everyone for these tips. I tried those tips before writing here.
But I have an announcement, my Dell Inspiron 3501 passed away today at 11 am, after about 20 days having to keep it on or avoid more than 6 hours off, otherwise I would have to take the battery out of the BIOS and disconnect the battery from the notebook for more than 3 hours.
Today, even after trying several times to do these procedures, it did turn on.
The cause of death was a short circuit in the motherboard and is now resting in Dell's heaven (or probably he...ll) with his brothers and cousins who can't stand 1.5 years of work.
Bye Dell! See you nevermore

I suggest you to follow the steps provided below, first to release any static power in the laptop that may be causing this issue:

1. Remove the battery and the AC Adapter from the laptop.
2. Press and hold the Power button for 20 seconds and release it.
3. Reconnect the battery, AC Adapter and press the Power button.
4. The laptop should power on if the static charge was the cause.

You can press the "Fn" key and the "Power" button to start the hardware diagnostics if the laptop fails to power on after following the steps provided above.

Replace bios battery, then press before you press power button hold down Windows + B. This will boot you into bios. Set the time and date correctly. Then it will boot fine! Just worked for me!