My car went for service and came back mechanical

On the 2nd April I took my car to these people for Engine oil service. My car only had 1 warning light, which was the due Engine oil service. When I picked it up after they said the service was done, it had a Drive train warning light and I showed them. the guy took my car in again to the back and came back after a few minutes saying I can drive home with that warning light but there's some crack on my spark plug or whatever. Something that didn't make Sense. I told him my spark plugs recently got replaced like 6 months ago and he just said I must book my car in again for Major service. This was a few minutes before they close business for the day and I drove home. On my way home my car got another warming light that I should stop because of transmission and cool down. I messaged these people for 2 days and NONE of them responded to my communication. I am now busy with lodging a formal complaint against them. I don't know what these people did to my car and it was my first and last time taking my car to them.