Welcome to the forum!
Okay here we go:
Turn on headlights and try starting - do they dim when you press the start button? Also, if you hold the starter button for, say, 10-20 seconds, do the battery wires get hot? If so, the starter is stuck - something you (or they) did related to the one-way is preventing the engine or starter from turning. Pull the starter cover and remove the intermediate gear - does the starter turn now? Perhaps one of the one-way bolts has come loose and is jammed.
If the cables don't get warm (and/or headlights don't dim), short across the two large lugs on the starter solenoid with a big screwdriver (ignore the sparks)... does the starter turn over now? If so, the solenoid is bad.
If not, then your battery is dead or the battery contacts are loose, dirty or corroded. First, remove and clean all mating surfaces on battery and cable lugs with a wire brush, then tighten with a wrench, not screwdriver. Does it start now?