I have downloaded the iso file and put it on a flash drive. After that i booted from the flash drive and the normal Ubuntu installation screen popped up. I clicked install now and it did everything right, but once I rebooted it will not come up. Why is this?
It can be caused for two different things:
One is that if you edited partitions, maybe you installed Ubuntu in a non-bootable partition. Then you will need to reinstall Ubuntu and make sure it's installed in a primary partition and that it's able to boot from there.
Other thing that can cause that is currupted .iso file of the Ubuntu Live CD/USB, so you will have to download it again and reinstall Ubuntu from the new .iso you've downloaded.