It seems like your save file got corrupted.
Go to C:\Users\admin\Zomboid again
Open the "Saves" folder
Open the "Multiplayer" folder
There should be a folder called servertest_player for example, depending on how your friend named his settings.
Delete that folder and try connecting again.
You will lose all progress on your character though.
Just tested it. You don't need mechanical skill. You need to have charger and battery in your equipment and stand on a tile that have power on. Then you right click either the battery or the battery charger and you should see approximate option to charge it, you can then observe the progress above your head or the progress on item bar.