I have tried to install Ubuntu OS from USB stick but failed. All installation process going well and at the end it shows to restart laptop after installation and when I click to restart laptop, it shows: "Operating System not found".
I have used several USB stickes, I have used "Rufus 3.6" and "Universal-USB-Installer-1.9.8.8" to make bootable USB stick, I have tried to install Ubuntu and Linux Mint but the result is always the same: "Operating System not found". Please, help. I need it for my semester exams :(

Laptop: Fujitsu Siemens Amilo Si 2636
Processor: Intel (R) Core (TM) 2 Duo CPU T7500 @2.20 GHz

I attached all the pictures of Boot Setup: https://rashidov.wordpress.com/2019/09/06/bios/

Important Note: Boot Device Priority does show Hard Drive. It shows only USB stick when I insert USB stick into laptop.
 
Is your "Boot Device Priority" set to CD/DVD ROM?

When I enter to BIOS in Boot section "Boot Device Priority" does not show Hard Driver. It shows only USB stick whenever I insert USB to the laptop. When USB stick in the laptop "Boot Device Priority" shows : USB Hard Disk.
and that is all. Please check the 5th photo on this link:
https://rashidov.wordpress.com/2019/09/06/bios/
 
Maybe this information will help you. So I insert the USB stick to the laptop (it contains Linux Mint for installing) and switched on Laptop and it booted automatically from USB and I entered to Linux Mint and then I checked information about Hard Driver in the section called "Disks":


Model: WDC WD2500BEVS-22UST0 (01.01A01)
Size: 250 GB
Partitioning: Master Boot Record
Serial Number: WD-WXHX06073***
Volumes: Filesystem - Partition 1 - 250 GB Ext4
Size: 250 GB
Device: /dev/sda1
UUID: 94d71065-bacf--*
Partition Type: Linux (Bootable)
Contents: Ext4 (version 1.0) - Not Mounted
 
When I enter to BIOS in Boot section "Boot Device Priority" does not show Hard Driver. It shows only USB stick whenever I insert USB to the laptop. When USB stick in the laptop "Boot Device Priority" shows : USB Hard Disk.
and that is all. Please check the 5th photo on this link:
https://rashidov.wordpress.com/2019/09/06/bios/
Maybe this information will help you. So I insert the USB stick to the laptop (it contains Linux Mint for installing) and switched on Laptop and it booted automatically from USB and I entered to Linux Mint and then I checked information about Hard Driver in the section called "Disks":


Model: WDC WD2500BEVS-22UST0 (01.01A01)
Size: 250 GB
Partitioning: Master Boot Record
Serial Number: WD-WXHX06073***
Volumes: Filesystem - Partition 1 - 250 GB Ext4
Size: 250 GB
Device: /dev/sda1
UUID: 94d71065-bacf--*
Partition Type: Linux (Bootable)
Contents: Ext4 (version 1.0) - Not Mounted
Everything there looks good.
 
Thanks to all who tried to help me.
I have tried everything but the result was the same and in the end I decided to buy a new HDD and the problem solved. Additionally, I bought SATA USB cable and now I am using my old HDD as an external Hard Drive.

Thanks to everyone.