If you're struggling to find an email then you know was sent to you, then the first place I would look is in your spam folder. Sometimes emails find their way straight there, even if they don't look like typical spam. Instead, the emails get filtered out by your email provider because they suspect them of being junk mail that you might not be interested in.
The technology that detects junk mail is usually pretty accurate, but on occasion it does get it wrong!
One common problem is that you may have deleted the email by accident. Luckily, you can check for this.
All email servers are different, but if you're using Gmail, just put the name of the person/company you expected the email from and then click the grey arrow next to the blue 'search' box.
Here you can give more information regarding keywords etc but you also want to click on the arrows in the grey search box and change the search options to 'mail & spam & trash' that should cover all the places that it may be hiding.
Sometimes emails do get lost though (or people don't actually send them) so it may be worth contacting the sender and asking if they can re-send it to you.