As Factor-fiction suggests, it's quite likely that your email provider's spam filter has dumped the email into a 'trash' folder. (It's estimated that well over 10% of all genuine emails are wrongly classed as spam by such filters. I once had to try three different email services before a friend in America could receive my emails without his provider blocking them; I then found that the email service I was using was blocked his emails to me!).
If you use an email client (such as Outlook Express, Thunderbird or Windows Live Mail) log into your email account via your provider's webmail access (i.e. via your browser, rather than via your email client). Then look for a folder labelled 'Trash', 'Spam', 'Quarantined' or something similar. The missing email may well be in there. (If it is, there should be a way of marking it as 'not spam', so the problem doesn't happen again). However you need to be aware that many email services delete items from such folders after perhaps only 7 days (but some keep suspect emails for far longer).