For my instructions, download these, you will end up using them:
1. Android drivers for windows, I would prefer Universal Naked Driver (UND)
2. Android debugging tools (ADB), I would prefer minimal ADB
3. TWRP recovery image: http://www.teamw.in/project/twrp2/90
download the last version you remembered using or just use the latest openrecovery-twrp-22.214.171.124-maguro.img
Install 1 & 2 in order. You may need to have the phone boot to recovery to install #1. Installing the drivers in Windows XP/7 will be easier, Windows 8/8.1 requires an advanced step to disable unknown driver enforcement.
For #2, if you have ADB/Fastboot already, skip this step. If you don't, then you may need to install it while in recovery too. Install it to a folder you are familiar with, I would choose C:\ and make an Android folder.
After you have both installed, boot your phone to bootloader mode. When it is shut off, hold VOL + and - at the same time then press the power button for all 3 simultaneously. It will boot into bootloader mode, where you see the android and your Galaxy Nexus revision/software details.
Move the TWRP recovery img file to the location of your ADB folder. For my example, I will assume #2 (adb tools) are installed in Local Disk (C:) and the android folder, so C:\Android
From here you would use the command prompt/terminal start > run > cmd or start, find program Command Prompt.
In the terminal type cd\foldertoTWRPimgfile\ or in my example, cd\android\
Next, type fastboot flash recovery openrecovery-twrp-126.96.36.199-maguro.img
my example would look like:
C:\android\ fastboot flash recovery openrecovery-twrp-188.8.131.52-maguro.img
it will show a progress indicator, then OK'd when it is finished.
You can then manually boot to recovery in fastboot, hit a volume button on your phone a few times until you see RECOVERY, then hit the power button to load it. You should now be in TWRP and be able to restore your backup, hope it is still there. Since it is likely that you were last using an older version of TWRP for MMuzzy, you might notice differences if you flash a newer version of TWRP.