I put my LAPTOP to sleep mode, while the USB flash drive is plugged in.

I pressed the power button to resume my laptop from sleep, the little blue lights on the machine indicated that it did wake up, but the screen was not displaying.

I pressed the power button again to turn it off.

And then turned it on again, the screen shown the BIOS screen as the following;

DELL
Inspiron 1525 series
BIOS Revision A16

After that, the screen turned black, didn't show ANYTHING.

But I can hear the Windows sound (Windows log in sound).

I unplugged the power cord, took the battery out, and held the power button for 60 seconds,
then put the battery back in, plugged the power cord in, and turned on the laptop,
the screen was still not working.

I powered on the laptop and pressed F2 continuously to go to the set up page.
The blue screen of set up page flashed up for a second, and then the screen went dark again.

I repeated these steps, still got the same issues.

Finally, I shined the flashlight on the screen, I was able to see all the processes since the machine started working, the BIOS screen, Windows logo, log on screen, and finally my desktop, but all these were very dark, without the flashlight I couldn't see these.

My laptop is Dell
Intel® Pentium® Dual CPU T2390 @ 1.86GHz 1.87 GHz
Windows 10

I don't have an external monitor to test this laptop screen.

How to fix this problem? Hi
Thanks for all the detail. It is mostly likely either the back light for the display or the inverter which supplies the high voltage for the back light.

*****Note high voltage, not something to play with if you are not technically trained.*******

Try turning the brightness up by pressing the F5 key. If the display is still dim then you will need someone with the gear to test the inverter and if that is ok to replace the display.
Is it ok to replace just the back lights or I need to replace the whole display? Hi

Easier to replace the whole display . The INVERT er is a separate part and quite cheap and is the most likely part which has failed the florescent tubes don't normally fail so quickly they go dim over time. Before jumping in and replacing the screen, you can check the backlight by shining a bright light onto the front of the screen, if it's just the backlight that's at fault, you should still be able to see some sort of faint image on the screen. I'd also try disconnecting the screen from the motherboard and hooking up an external monitor to ensure that doesn't exhibit the same behaviour as the internal screen (disconnecting the internal screen will often force the laptop to use the external monitor as its PRIMARY output device). I'd also recommend purchasing a screen from somewhere that allows you to return it if it turns out to not be the solution, better than ending up stuck with a screen that you don't need!+ 1 ! !Here is a youtube video of what is involved in disconnecting the laptop display as Cameron suggested.

https://www.youtube.com/watch?v=spfHa_FRq6c

As you have already done the bright light test and can see the laptop booting.
Also an external monitor will work without going to the trouble of disconnecting the display but you said in the original post you don't have an external monitor to use for testing.
Where you go from here depends on whether you want to fix it yourself after seeing the video or have a tech do it.