I can see a couple of options but they depend on you being able to get to a command prompt with a working internet connection.

1. Install another desktop.


2. Burn a new USB installer from command line.

Install another desktop

This does not guarantee a working desktop though. Your problem might be something related to video card and also present in other desktops. Drop to a command line and start of with this:

sudo apt-get update

and then pick ONE of these (to install that specific desktop):

xubuntu-desktop 

lubuntu-desktop 

ubuntu-mate-desktop

I only listed a few (you could also install kUbuntu but for just getting a desktop to be able to make an installation disk might be overkill).

Burn a new USB installer from command line

Download Ubuntu from command line with

wget http://releases.ubuntu.com/14.04/ubuntu-14.04-desktop-amd64.iso

(change the url to a release you want. see http://releases.ubuntu.com and the directories below that for what you need). It will show something like this:

~/Downloads$ wget http://releases.ubuntu.com/14.04/ubuntu-14.04-desktop-amd64.iso

--2015-02-28 21:09:59--  http://releases.ubuntu.com/14.04/ubuntu-14.04-desktop-amd64.iso

Resolving releases.ubuntu.com (releases.ubuntu.com)... 

91.189.92.151, 2001:67c:1360:8c01::1e

Connecting to releases.ubuntu.com (releases.ubuntu.com)|

91.189.92.151|:80... connected.

HTTP request sent, awaiting response... 200 OK

Length: 1010827264 (964M) [application/x-iso9660-image]

Saving to: ‘ubuntu-14.04-desktop-amd64.iso   6%  58,05M  1,36MB/s   eta 7m 47s

and then "dd" the ISO to the USB flash drive. Pay extra attention to the device name. If you mess this up you could overwrite your current install... I tend to use dmesg:

[26005.823077] sd 7:0:0:0: Attached scsi generic sg3 type 0

[26006.953612] sd 7:0:0:0: [sdc] 30299520 512-byte logical blocks: (15.5 GB/14.4 GiB)

[26006.954291] sd 7:0:0:0: [sdc] Write Protect is off

[26006.954298] sd 7:0:0:0: [sdc] Mode Sense: 23 00 00 00

[26006.954912] sd 7:0:0:0: [sdc] No Caching mode page found

[26006.954918] sd 7:0:0:0: [sdc] Assuming drive cache: write through

[26006.972919]  sdc: sdc1

[26006.976297] sd 7:0:0:0: [sdc] Attached SCSI removable disk

So that would be "sdc". This will burn it (skip the 1st one if you are still in Downloads, skip the 2nd one if it is already installed):

cd ~/Downloads

sudo apt-get install gddrescue

sudo ddrescue -d -D --force ubuntu-14.04-desktop-amd64.iso /dev/sdc

Results:

GNU ddrescue 1.19

Press Ctrl-C to interrupt

rescued:    63504 kB,  errsize:     230 B,  current rate:     196 kB/s

   ipos:    63504 kB,   errors:       1,    average rate:   12700 kB/s

   opos:    63504 kB, run time:       5 s,  successful read:       0 s ago

Finished                                     

and an image of the freshly created USB ISO: