Welding on the frame can fry electronic components unless you remove them or unplug them beforehand, because high voltage and currents can pass through them. Depending on where (and how good) your welding ground lead clamp was placed on the frame, nearby components might take a hit... or in case of a poor frame ground, welding current might take alternate paths through unknown things.
Since all power is dead on your bike I'd begin by cleaning and snugging up all of the battery cables, the solenoid connections and the ground cables to the frame and the starter mount. Look for any overheated or melted wiring on the bike... Check the battery to make sure it is still alive and healthy. Check the solenoid and starter motor. Check fuses and fuse sockets for overheating damage or loose fuses. Check all of the indicator light bulbs (neutral light in particular) to make sure they aren't blown and don't forget the tail lights and headlight bulbs. Check the diode inside the connector box under the front fender. If everything is still dead, unplug and inspect all harness plugs for damage then check the ignition switch.
Beyond thar' be dragons... You'll need a multimeter and the service manual to safely get past them. You can get a copy of the service manual
HERE if you haven't one yet for reference. The manual will help you keep your money in your wallet where it belongs.
Wiring diagrams are in the back... refer to the Battery & Starting section, Ignition section and the Lights & switches section for initial troubleshooting beyond the basics.
Let us know how it goes if ya can... it might be something simple and basic.
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