I'm not sure where to put this, as it's not really an installation upgrade issue, nor does it fit in most of the other forums.... basically my sh*t just blew up.

One day the surveys are all chugging away fine, the next (today) I go to log into the system and I'm getting a 500 error (or setJsVar is not defined error, depending on which browser I'm looking in). I've done nothing to the server, no added surveys, no dev of any kind. Data still seems to be there, and the front page (choose language) is still there but anything deeper is unreachable.

I could really use a few ideas on how to address this issue.

Thanks in advance.

Doesn't sound good. Let's see. I see no chance that things worked well one day and then nothing works without ANY change in the system. We will now have to find where the problem is.

What version of Limesurvey are you using?
Did you do an update between yesterday and today? (I assume the answer is "no", as you said you did nothing, but...)
Who has access to the server?
Where is the server hosted? Who is responsible for hosting? In most cases when this happens some admin made changes to the system. So we need to rule that out first.
Do other web apps run on the same server? Are they doing fine?

Version... I'm not sure. I believe I'm one update behind, but I can't get in to check the actual version number
Updates... you assumed correctly
Where.. Site5 hosting (off site, 3rd party)
Other apps ... I have some php pages that are pulling results directly from the db for some visualizations I have and that is all working fine.

It appears that it is something to do with the authentication wall. The index page (with the language selection drop down) pulls up fine but anything beyond that, admin page or the surveys throws the 500 error.

OK, no changes to LS. This means the change must have happend on the server.

I would get in touch with my hosting company and ask if they did any changes to the server. You have a pretty clear time window of about 1 day. It should be possible to nail this down.