Someone has already given you a guide on how to change the password via grub params, however that might not be necessary. It's important that when you ask these questions you provide the exact command you're using and the exact things you changed, what I assume happened is that you removed yourself from the wheel group (which in Mint might be listed as Admin privileges), this is important because the sudo command (unless you've customised it) only works for members of the wheel group. Then you said you tried to access the root via terminal and it told you you didn't have permissions, I'm 99% sure that you tried something with sudo here, which will no longer work because your user is not an admin anymore.

So are you screwed? Nope, you can access the root user without the need for sudo. Most people when they want to have a root shell use sudo su, in that command su is the actual part that lets you login as root (in fact su is short for ~~Switch~~ Substitute User, and you can switch to any user using it. Fun fact sudo is a short for ~~Switch~~ Substitute User and DO, so that you can run things as root without needing to login as root). As you might have already guessed just running su and putting the root password should work. Then why do people use sudo su? Because sudo su will ask you for YOUR password, whereas su asks you for the ROOT password (which in most servers is different, but most home computers is the same). You set the two of them when installing the system (in fact it's very likely that you ticked a checkbox that said something like "use the same password for the root account").

Edit: The correct therm used in the su manual is Substitute not switch as someone mentioned in a reply.