Operating System Not Found

Well with this you will either need a Dell recovery disk or an OEM/Retail Windows 8/8.1 installation CD depending on the version that was installed. (The Win 8/8.1 disk will read the CD-key off the hardware and allow you to reinstall windows that way.)

Well in this case what you are seeing is that the firmware is looking for something to load (Windows/Linux etc) and it can't find anything on the connected media to load. Hence the message you got saying an operating system was not found.

The installation disks can be used to troubleshoot the issue along with reinstalling the OS. But without one not even that can be done. As your issue could be a simple as a missing boot loader. (Which is called right before loading the OS it can either load the OS or select from a set of options either another OS or loading an OS with a parameter like booting Windows in safe mode)

Hey, I had the same problem, I tried resetting my laptop initially but it got stuck after putting it off and on manually, it displayed "an operating system not found, disconnect all drives that do not contain an operating system. press cntrl + alt + del to restart".so my thinking was that something was wrong with the os since the system wasn't recognizing it. I use an acer aspire E-5 571, so I tried the acer erecovery management by pressing alt F10 as soon as the laptop starts to load but nothing happened. My laptop had previously been running on the legacy boot mode, so using F2 as soon as the laptop boots I changed the boot mode to UEFI and tried the erecovery management this time it just got stuck, on the very first screen. Then I put an operating system disc image (iso file) into a flash drive, changed the boot priority order, allowing the drive into the first spot on the list, then rebooted in UEFI mode. It loaded a windows mini-window having install now and repair your computer. Now I had watched a lot of youtube videos based on the problem, used the command prompt so do a lot of things, make partitions active, assign, nothing worked!! And when I try installing, it doesn't work in normal install, saying I should do so when windows has loaded (I can't even get my laptop to boot up!!) in advanced mode, it says windows can't be installed on any of this partitions. I couldn't reset it from repair your computer> troubleshoot> reset, because it says the drive where windows is installed is locked (actually supprising because at this time I doubted if there was still windows installed on it after everything I had been through). At this point I was tired, exhausted, frustrated and confused. After thinking about it for a while, I resulted to unconventional means, I did it my self! So this is how it goes, I boot up the laptop in UEFI mode with the flash drive(I had formatted it before putting the windows file in it, not as an iso file but all of its content directly and with no other file in the flash), it started the mini-window, now following repair your computer>troubleshoot>advanced options >command prompt
(commands in bold)
Diskpart enter
list disk enter
select disk (the main drive) enter
clean enter (this wipes everything, clean slate or so it seems)
Then I did a fresh install of windows with install now>(skipped the key part) advanced>(there was only one partition) I installed windows here.It completed the installation and everything went to normal. Note I had previously backed up my files and using this as a DISCLAIMER, you do this at your own risk, although I hope it works for you too.