Ok, I realize I'm opening myself up for some ridicule, but here goes ...
My son's '03 Passat 1.8T sat all winter and the battery was dead, so I went to jump it with my daughter's '05 Escape. And yes, I made the bone head move of mixing up the cable connections on the donor vehicle! Luckily we didn't have an exploding battery, but the Escape ran like crap afterward and the Passat would crank but not start. (A battery disconnect and reconnect on the Escape reset the ECM and luckily everything was fine. Whew!)
Wading through the forum here, it did not sound good for the old Passat! Visions of troubleshooting obscure electrical issues with ECU, CCM, TCM, relays, fuses, coils, etc, etc, etc about gave me nightmares.
So, here's what I found and how I fixed it:
First - I had the battery tested and charged.
Again, the '03 AWM would turn over but not fire, so I pulled a plug, and tested for spark - nothing.
I checked all of the engine-related fuses in the interior fuse panel again and they all looked good. (28, 29, 32, 34, 43)
Next, I plugged in my OBD scanner and got an error indicating it couldn't connect.
Now getting pretty mad at myself for the dumb mistake!
Read some more forum posts about similar 'cranks but won't fire' problems and someone suggested checking the ABS fuse #7 because that apparently supplies power to 28 and 29 and maybe others? Pulled it out and it looked fine at first glance, but ohm'd it out and sure enough it was blown. Replaced that and the old boy fired right up!
Everything seemed good, but the Monsoon radio was dead. Tried the pulling fuse 37 and resetting the radio, but that didn't fix it, so I pulled the radio quick and found the 10 amp fuse at the back was blown. Replaced that and the radio came back to life.
All is good for now! (Fingers crossed)
So ... if you hook up the jumper cables backwards on your B5, pound your head against the wall for being so dumb, and then check fuse #7.
Let the ridicule commence!