My son thought he had a flight today SFO-PDX which he booked a couple months ago, or thought he had. Last night he realized he hadn't gotten the email reminding him it was time to check in for his flight, and when he went on line he discovered he couldn't check in. It turned out there was a problem and the payment never went through, so while he had made the reservation, he was never ticketed.
Fortunately, after call-waiting-hold hell, Alaska was able to re-book him for an earlier flight today and even honored the original fare (!).
Pro Tip to Remember: There are three phases to flight bookings: Reservation, Ticketing, and Issuing the boarding pass. When you book online, you should get a confirmation of the booking, then after payment goes through you should get a ticketing confirmation with your e-ticket number. (United has always sent me two separate emails.) If you don't get an e-ticket number, you need to follow up (I've seen my e-ticket not show up until the next day).
The boarding pass phase doesn't come until you check in for your flight. Usually this is 24 hours prior, but these days you may have to check in in person if the agent needs to verify documents.