If they are having sex at 14 they won't be the first. I fully agree it's too young ideally - I think I have more liberal views about sex on here to most and even I have always told my daughter I would strongly prefer her to wait till 16 at the very youngest, as I think sex is a big thing to process emotionally as an adult, yet alone as a child....
But the reality is, by 14 (or younger in some cases) many people have pretty much the body of an adult, and moreover both their libidos and the risk taking parts of their brain (I forget all the technical terminology) are at an all time high in adolescence - they are actually doing what comes naturally, and though your daughter may well respect you and wish she could follow your instructions, that part of her is having to battle a massive weight of evolutionary biology....
Punish and berate her and all you will do, in my opinions n, is weaken her respect for you. (Unless you have a highly conditioned child who is very scared of you - and if that was the case I doubt she would have risked sneaking him in in the first place).
Much better to talk, and listen. Explain to her your reasons for not wanting her to be sexually active at this stage (IF she even is). And acknowledge that there are lots of forms of intimacy that she can safely practice which might be more appropriate for their age, and which are not full penatrative sex....
But if she is determined to have sex she will, and I would actually prefer my underage daughter to have sex in the safety of our own home, where she is far less likely to get hurt, than in a bush in a park somewhere, if those were the only options......
And you said that going shopping was obviously cancelled - why? I would think you really need to be spending quality time with your daughter and really talking to her at this point - and nurturing your relationship with her so that she will be more honest with you in the future. The more she believes you love and value her, the more likely she is to care about your advice......
It must have been horribly shocking though, and I see why you are very stressed out. Good luck!