Problem with template cropped up

[guruchild]

My site has been running SMF for several months now. Haven't made any major changes in the past few weeks... seemingly sometime last night the site began giving the following error... what I know is that another member who had logged out was already getting the error. I wasn't getting the error until I logged out. Thus, for some reason, now, if someone is a guest presumably they get the error...

8: Undefined index: ID_GROUP
File: /www/1/Themes/default/BoardIndex.template.php (eval?)
Line: 151

I seem to get this error regardless of what part of the forum site I try to access, if it's something else it would just say whatever instead of BoardIndex.template.php.

I've been digging around some but I'm pretty clueless about this one. It is extremely frustrating to know things aren't functioning quite right for strange reasons. Nothing on the server was changed to my knowledge. Please advise.