The Monero GUI has several options for restoring your account. When starting the GUI for the first time, you will be presented with the following options:
Select the middle one, "Restore wallet from keys or mnemonic seed." Of course, if you have the wallet file and know the password, select the last "Open a wallet from file" instead.
If you are restoring a wallet from keys or mnemonic seed, you will see this new page:
Select "Restore from keys" if you are restoring from keys, and select "Restore from seed" if you are restoring from seed.
Make sure to include a block height if possible. For instance, if you started using this wallet at block height 1,500,000, then enter a number less than this, for instance 1,450,000. Adding this block height will substantially speed up the restoration process. If you do not know when you first used the wallet, then leave this field blank.
If you are using MyMonero, you need to restore the account from keys, not from mnemonic seed.