At the terminal prompt, you can start your Desktop Environment with the command
startxEnter. Copy off everything you want to save to get it off the drive.
As to the LiveUSB you used to install Ubuntu, I would suggest recreating it after copying off all the files you need to preserve on that USB. Redownload the version of Ubuntu you wish to use from https://ubuntu.com/download/flavours or https://ubuntu.com/download and verify it was not corrupted in transit.
Once verified, open Rufus again; if it isn't version 3.8, I would download the current version and install that.
Boot selection should be set to Disk or ISO image. Click on SELECT to the right of the Boot selection field then choose the ISO file you just verified.
Partition scheme should be GPT.
Quick format should be checked.
Create extended label and icon files should be checked.
When it completes, boot your Ubuntu PC from the Live USB you just created, and reinstall Ubuntu (or the flavour you choose instead of vanilla Ubuntu).
When you reach 6. Allocate disk space pick the option to erase the drive and (re)install Ubuntu.