An increasingly common occurrence, given the intensity of the current market. Did they just reject your offer outright? If so, that means that your offer and the seller's desired price were most likely too far apart to even make it worth their while to make a counter-offer. Did they counter and you decided not to move up? Or did you get to a situation where there were multiple offers and the seller declared a "best and final" deadline, allowing people to improve their offers if they felt so inclined? In that case, if your offer was not selected, most likely the sellers went with another offer, that they perceived as better (either higher, or all cash, or both).
If the seller has accepted another offer, it's not likely you can do anything about it, unless you are prepared to offer a much higher amount and/or forgo some protections like the mortgage contingency (meaning that if your offer is accepted, and you were unable to get a mortgage, you would lose your 10% contract deposit - I really wouldn't recommend this to anyone unless you were 1000% sure your mortgage would be approved).
If either of the first two scenarios is the case, and the property is still available, you can always try going back with an offer closer to the seller's asking price.
Check out this recent Real Deal article on current market conditions - this is not a time to go it alone. An experienced real estate agent or broker will be able to advise you in preparing your offer and guide you through the process.