I have made an USB Drive, as live USB. But I am unable to boot from it. It has all required files. Assuming I don't have original ISO, how can I attempt to make it bootable again?
First, if the USB stick is not booting, I recommend erasing the whole USB stick and remake it again.
Download pendrivelinux or UNetbootin to create the Live USB again. The program will automatically wipe all the stuff on the USB stick and copy the files from the Ubuntu iso to the USB stick (and remember to download the iso from the Ubuntu site.)
If it doesn't boot, and it shows the purple screen before the computer blacks out, try changing the boot options.
Tap F6 when you reach the purple screen.
nomodset first. Try all other options of the options mentioned didn't work. Use ↑ and ↓ to switch options and press Enter to choose the option.
If the computer boots, you are done, not really. After you installed ubuntu, read this thread on the Ubuntu Forums and jump to the "How to permanently set kernel boot options on an installed OS (not wubi)" part to apply the boot option permanently.
If the USB stick still won't boot, read My computer boots to a black screen, what options do I have to fix it? to find the cause of not booting.