Came back to check your progress and find out you sent an email, bad move. As I said previously you need to make it 100% clear that this is a date, from what you say that doesn't seem to have been the case, however you have now moved it on to a different sceanrio.
The biggest mistake is that your email set a boundry, your success or failure was limited to a single response. You gave her the option to delay or avoid the issue because even if she wasn't going out she could say she was. If you wanted to do it via email (which in my opinion is never a good idea) should have been to say something along the lines of "do you fancy going for a drink after work some time, just me and you".
However all is not lost, I do agree that no additional reply from her of "can't tonight but maybe another time" might put a bit of a downer on the situation, however as I say she may not be fully aware of what you were asking.
Nothing left to do in my opinion other than ask outright for a date. Believe me the pain will be over quickly.