Could this also be " What did you write?"
That would be simply तुमने क्या लिखा
तुमने क्या लिखा *था।
था here shows that it is a past tense.
था (was) is the past form of है (is)
Yes, but in this case it says likha hai not likha tha ?
Ahhhhhhhh now I see
What I have written, I have written.