Installed Ubuntu 18.04 on a Dell Optiplex but after the withdrawal of the flash drive and pressing enter to reboot the computer, the computer screen froze just before the login appeared. The installation was repeated with five different flash drives with both Ubuntu 18.04 and Ubuntu 18.04.1. The same result occurred each time. After some research, it was discovered that the LightDM has been replaced as the default login display manager by GDM3 which apparently is a lot heavier. Although the Dell Optiplex is over ten years old it still meets the minimum requirements for the installation of Ubuntu. It will install and reboot after installation of all of the Ubuntu "flavors" [Kubuntu, Ubuntu Mate, Ubuntu Budgie, Xubuntu, and Lubuntu] as well as Ubuntu 16.04.4. If a computer meets the minimum install requirements it should be able to install Ubuntu. As a work around the LightDM was installed and made the default login display manager. That works as expected. Is the new default login display manager to heavy for some computers to load?
Received the following response from A Raghuram (araghuramindia) to the question (#672572) that I posed:
"See if the following steps help you:
1) Choose recovery mode from the GRUB menu
2) Choose resume normal boot from the recovery menu
3) If you are able to get the login screen,
after logging in, uncomment the line "WaylandEnable=
4) Then restart for a normal reboot.
I had the same problem with a non-Dell desktop computer and the above steps solved it."
This work-around also allows the computer to boot normally.