Normally you have to wait a few minutes for the transaction to be mined before it shows up in your account. Note that this can be as long as several hours, in unusual circumstances. In addition, sometimes your browser may not reflect the funds immediately, if the system is under extreme load, and you have to log out and log back in again.
If the Monero you are expecting is still not there after all of this try logging in using a different browser, and if it still does not show up you will have to contact the sender and confirm that they have sent it to the correct address. It is critical that the sender also provides you with the transaction ID for the transaction, so that you can confirm it has been mined by checking that it has a "Block" number on a block explorer such as

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