Respect her decision but don’t take it personally. She may just happen to be in love with someone else or you may not be her type. Don’t be upset about it, however don’t be suddenly very different with her than you were before.
I am not in favour of asking, you have to be able to gauge it with eye contact, smiles and then do the lean-in. You cannot really kiss someone by stealth, or even if you can, you shouldn’t.
Something to exclude first
Always worth as well checking you don’t have a breath issue. Just to exclude that as a reason why, in the absence of any more obvious reason.
The onus is on the girl to help you to create a situation where you can repeat it if she actually would like to be kissed and was simply taken by surprise.
No biggy, you’ll see
But the main thing is, don’t treat it as anything major. It is a minor thing, really minor, as you will know for sure in due course even if your cheeks are burning today. No biggy; it is a smally of the smallies. You may feel that you have lost face, however, unless the person concerned was glanding some powerful acid that she herself was immune to, that is not likely to be the case in actual fact.
It depends. If someone had said they were interested before but you had a partner then, and gave that as a reason, then you could say something like “when you told me you were interested before, I had a partner and didn’t want to be disloyal, but that hasn’t worked out. It’s over. So if you are still interested we can go out”.
But if you are simply asking out of vanity then I don’t think it would be very nice to get the other person’s hopes up only to dash them again.
The golden meanie
In between those two extremes there lie all kinds of other scenarios, but I wouldn’t ask that person unless you are ready to go if they say “yes” and if you have a reasonable reason why you didn’t say yes before, such as that you had a partner then, or you were dealing with a stupid infatuation, or that you didn’t realise you has so much in common with the person. Or you thought they miht have been just joking around and now you hear they have a serious character and were probably not. Or you thought they were not going to be living in the area for long, or you weren’t going to be, and that’s changed. These are all reasonably valid reasons for changing your mind about someone’s approach.
“I said no earlier because I thought I had a chance with someone better than you” is a bad reason. I wouldn’t bother asking in those circs. Or “I didn’t know you were rich” is another bad reason.
Maybe the first step is to be honest with yourself as to what reason or reasons made you turn this person down before and what really changed. Then you can see if you can sell that reason to the other person.