This constitutes barrier failure, and you have been exposed to the semen. If you feel as though you've missed a period, take a pregnancy test. Otherwise, in future to prevent the same thing happening, ensure you are using the right size condoms (he might think he needs XXL condoms.. but maybe not), and that your partner is withdrawing from your vagina immediately after ejaculation or as soon as he feels he is losing his erection.
I once had to fish a condom out with forceps in the emergency room, don't let that be you!
Actually the above is incorrect. You had a condom failure. Very possible that sperm could have leaked out putting you at risk for pregnancy. Hopefully not. But do watch for your period this next time and hopefully it will show up. good luck