Elaborate first on what did you do so that we can give you an accurate answer since if you bought some btc in coinbase then the first thing you need to do is to see on your balance if your purchase has been successful and after that if still no BTC show then much better to contact there support or find the most convenient way to contact them since most provably there's an issue happened when your transaction done. And lastly check the site if you really use the legitimate site since there are so many phising site which used the name coinbase just to scam people here.