Loss of operating system of my laptop(Dell)

"Restore Windows 8 using the Factory image restore method in Dell Backup and Recovery"

http://www.dell.com/support/article/us/en/19/SLN155115/EN

"How to enable boot from DVD option with UEFI boot mode enabled. (Windows 8, 8.1 & 10)"

http://www.dell.com/support/article/us/en/19/SLN142679/EN

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Or:

"Create installation media for Windows 8.1"

http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media

A similar Tutorial which may also assist you

"Create Installation Media for Windows 8.1"

http://winsupersite.com/windows-8/create-installation-media-windows-81

In the first screen of this very simple wizard, you need to choose the following:

Language. English – en-us if you're in the United States.

Edition. Windows 8.1 (Core), Windows 8.1 N, Windows 8.1 Pro, Windows 8.1 Pro N, or Windows 8.1 Single Language.

Architecture. 64-bit (x64 ) or 32-bit (x86).

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"How to Create a Bootable USB or DVD with a Windows 8 or 8.1 ISO"

http://www.eightforums.com/tutorials/2227-create-bootable-usb-dvd-windows-8-iso.html

   Warning

This will not work to install Windows 8 or 8.1 for UEFI. For that, see this tutorial below instead.

UEFI Bootable USB Flash Drive - Create in Windows

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How to clean install:

"How to Do a Clean Install of Windows 8 or Windows 8.1"

http://www.eightforums.com/tutorials/2299-clean-install-windows-8-a.html

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Whenever I  start my laptop I receive a statement like this, 'An operating system was not found. Try disconnecting any drive that do not contain an operating system. ' I am so confused about what to do.  I can't even start or run my laptop.  It's very important to fix this problem as soon as possible.  Please help me do so. I request you to a solution to this.  For more information my laptop is of Dell company.  Thank you.

i lost thi password

I can't help you with copying an ISO from a mobile to a USB.

Download the ISO on a Windows 7 or above computer or laptop and make the Bootable installation media the correct way.

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Also:

Follow these Dell troubleshooting steps:

"Computer Will Not Boot to Microsoft Windows 8 or Windows 8.1"

http://www.dell.com/support/article/us/en/19/SLN115652/EN

Table of Contents:

1. Specific Windows 8 No Boot Issues
2. Verify the Computer Finishes Initial Power-Up (POST)
3. Repair your Windows 8 Operating System
4. Check for Specific Error Messages
5. Run a Computer Diagnostic: If you are unable to boot the system to access the Dell Online Diagnostics, use the Dell Pre-boot System Diagnostics.
6. Rest the BIOS to Default Values
7. Unplug All External Devices
8. Virus and Malware Troubleshooting
9. Restore the Operating System to Factory Settings

"What Dell diagnostic tools can I use to fix hardware problems?"

http://www.dell.com/support/article/us/en/19/SLN114988/EN

"Resolve Hardware Issues Using ePSA or PSA Diagnostics and Guide to Error Codes"

http://www.dell.com/support/article/us/en/19/SLN133583/EN

Actually my laptop is not in the condition of starting and running.  Hence,  I have downloaded windows 8.1 iso file from Microsoft website on my mobile so that I can copy it in USB pen drive and then connect it to my laptop to get it installed on it. But I am receiving error that 'couldn't copy file ' while copying it from mobile to USB. Can you please help me sir?

Do you have a USB flash drive or external hard drive plugged in?