Bummer. So was there output initially before changing the battery ? Normal default user preset would be 1 (Marshall patch) after battery change (as all presets are lost when RAM power removed). Normal operation it powers up with the last patch selected (e.g. in may case 112). Do you have any midi pedal for it ? (they shipped/came with an ADA MXC, 1 expression CC pedal and a quad switch). So does the system edit button (far right) not work either ? (i.e. none of the front panel buttons work at all, and did they work at all before battery change ?).
MJMP may have some ideas to check (he's our "under the hood" guru), and there are some other members also with good tech skills and experience.
One thing to try is remove, check/clean, reintsall the EPROM (stores the factory patches and firmware) located front right corner IIRC, should have a sticker on it with the firmware version, last EPROM update was v1.41. Pulling it out and reinstalling won't hurt anything (there's instructions in the mod and schematics area I think) but it's not hard to do (although take electrostatic precautions (power lead plugged in but power off, touch the case to ensure you have same potential difference)).
I'll PM you my phone number if you want to call, happy to chat and help more if I can
Cheers R