That's a physical characteric of the disc. You can't change it. You can't change a cd-r into a cd-rw nor the other way around. That's exactly the same "can't" as in "you can't change a banana in an apple". It's physically impossible.
So it's unclear what you did. Maybe the word is "multisession". Then you can't change it back to "single session".
Moreover, I've never seen such a question from Windows when inserting a blank cd. But that might be because I have burning program installed.
Just use another cd and give the right answer now.
I have Windows 7 OS and have dvd-rw drive in it.When I inserted a blank cd(cd-r),I was asked to choose whether to make it writable or rewritable,so I choosed to make it rewritable.But when I wanted to burn some mp3 audios from windows media player(to change it to cda format),it asked me to insert a writable cd(not rewritable).I tried the (above mentioned)cd but was again asked to insert a writable one.Now I want to convert this rewritable made cd to writable.Please help me to convert it to writable or else tell me any other ideas that is suitable to convert mp3 to cda on rewritable cds...