No reverse in 2006 hyundai sonata

What is your question?

I'm not sure if my transmission is not good at all., there is no reverse but no check engine on my dashboard and theres no debris either when i tried to drain the fluid. I have'nt done to fill it coz im waiting for some expertly advise also how much exactly i have to put back in so far the drained fluid equals to 1 in a half gallon. Please send me a further instructions and thanks for your help.

What makes this problem better or worse?

No reverse but no engine check

How long has your 2006 Hyundai Sonata had this problem?

Last week