Workshop forgot oil – engine damage!?

Hello, with my car the cylinder head was changed. So probably also the oil was drained. When I got it back everything ran. But after some time (about 2000km) a clattering came, which became stronger and stronger. Now I drove again into a Toyota workshop and they have the diagnosis : engine broken!!! Why? They probably did not fill in any oil before. The Toyota workshop tipped in 3 liters (in total only 3.5 liters go in). But why did we not have a warning Because the plug was pulled off. Now the question is how do I get to the workshop? Toyota recommended me to have an expert look at it…… To do this the whole engine must be dismantled. To see what now has taken damage can someone help me. Does someone have similar experiences?