Welcome to the forum, Uncle Rico! So sorry this has happened.
You asked if other people have experienced the same... Just a short while back, someone lost most / all of her fish due to the city spraying for mosquitoes. So, yes, it does happen.
Be sure your pond is situated so that water cannot run from the lawn and flow into the pond. If so, build a berm or barricade of some sort to keep contaminated water out of your pond.
If you continue to use this lawn service, make sure they do not use chemcials again around your pond.
Write to the manager, and the main franchise office (a nice, polite letter), to let them know what happened, ask them to take steps to prevent his in the future to you or other clients, and ask them to replace the fish and tadpoles.
Then, as advised, empty and clean your pond, refill it, stock it with cheap "feeder" goldfish, and - most important of all - KEEP ON PONDING! We'd love to see some photos, too!