Ubuntu 16.04 running on Dell Optiplex 7050. Xubuntu desktop installed.
I did not disable UEFI secure boot during initial install. However, I went back and disabled it so that I could run VirtualBox. I did enable legacy ROM support. Upon logging back in after disabling secure boot, I could no longer connect to the internet. My wired network shows connected, but I have no internet access. Please help
In theory, neither disabling Secure Boot nor enabling BIOS/CSM/legacy-boot support should affect networking once Ubuntu has booted, with one possible exception: If you've switched the actual Ubuntu boot mode from EFI to BIOS, that might affect hardware initialization, which might affect the ability to use the network device. To completely switch boot modes would require installing a BIOS-mode boot loader, though, and you haven't indicated that you've done this.
You can verify your boot mode by looking for a directory called /sys/firmware/efi. If it's present, you've booted in EFI mode; if it's absent, you've booted in BIOS mode. You might make this check, on the off chance that you've completely changed boot modes. Alternatively, you can simply disable your CSM. It's almost certainly doing you no good, and could be slowing down the boot process.
Although Secure Boot is even less likely to be related to your problem, there are ways to enable it and use VirtualBox. There are several questions and answers on this site that address this issue, such as:
Overall, my suspicion is that your networking problem is unrelated to the changes you made. Maybe you're experiencing a temporary network glitch, or your network's DHCP server has crashed. If I'm right, the timing of the problem appearing when it did is coincidental, and you should look elsewhere for a solution. I'm sure there are questions and answers on this site that address such problems, but I don't follow that topic closely, so your search on this topic is as good as mine.
