I completed installing Ubuntu alongside my windows computer, but I am not sure how to run Ubuntu. I have tried restarting my computer several time and reinstalled Ubuntu too.
Follow the below steps:
1) Go to BIOS menu. You can do this by tapping continuously Escape as soon as your computer restart. You will then get multiple boot options and then select Boot Menu either using arrows key or the press the Function Key mentioned next to it.
2) In the boot menu, go to advance settings->security and disable "Secure Boot".
3) Also change the OS Boot order by keeping Ubuntu as first and then Windows. Save and Exit.
Now you should get the boot menu with Ubuntu and Windows as option.
If this still doesn't work, try disabling "Fast Start Up" in Windows(assuming you have Windows 10). You can follow instructions here.