How Do I Revert Back To The Default Trisquel 7 Kernel?

If it is not already installed, you first need to install, with the package manager, "linux-image-lowlatency" (Trisquel 7's default kernel, currently at version 3.13.0.137) or any other kernel you would prefer (maybe "linux-image-generic"). Then, you need to reboot on that kernel, choosing it in GRUB's "Advanced options". To enter those options, GRUB will ask for a user name ("grub" is the answer) and a password, which written at the very end of /etc/grub.d/01_PASSWORD. I advise you to get rid of that useless password by following the steps I gave yesterday: https://trisquel.info/forum/after-installing-trisquel-7-dual-boot-ubuntu-grub-asking-password-boot-ubuntu#comment-124672

Once you run your kernel of choice ('uname -r', executed in a terminal, can confirm you the version you run), remove all the newer kernels using, again, the package manager. I like the "Synaptic Package Manager" (in the "System Settings", a graphical interface to APT. A way to list all packages installed from jxself's linux-libre repository is to click, in the "Synaptic Package Manager", on the "Origin" button (in the lower-left corner of the window) and then on "Local/main (linux-libre.fsfla.org)" above. Select them all, right-click on the selection and choose "Mark from Complete Removal" in the contextual menu that pops up. Finally, click on the "Apply" button and confirm.

Optionally, you can remove the repository from the "Other Software" tab of "Software & Updates" that you can either access from the "System Settings" or from the menu Settings/Repositories of Synaptic.

I wasn't able to access GRUB (i'm not sure how?) ... I just did "sudo apt-get purge" on the linux libre kernel with the help of "uname -r" to remove that version of it. Apparently I still had the lowlatency kernel on my machine, so after rebooting I checked my kernel and it was switched. So, in short I basically used like 10% of the info you provided. Thanks for the help though :)

It turns out the linux-libre kernel was the issue! I'm using a thiunkpad sitting on a dock. My monitors were plugged into the dock. It seems the linux-libre kernel doesn't like the dock when it comes to Trisquel 7. When I was using Trisquel 8 beta, it worked. I think I have a bug report! :)