Where to start looking....

You guys are doin' fine without me, LOL.

So you've now got the battery charged and tested for certain.

It's possible the tender itself has gone south.

Next, disconnect the ground cable and put a small 12V test lamp in series. If you don't see a light, take your multimeter and put it on whatever scale is the largest range of current (if you post your multimeter brand model, we can look it up) and put it in series with the ground cable. If you read no significant current, drop the amp/ milliamp scale down a range or two and post the reading. Make sure EVERYTHING in the car is shut off, and if you have a hood/ trunk/ glove box light make sure those are all turned off.