You can restore a wallet by either using the keys, .keys file and the password, or the 25-word mnemonic seed.
Restoring from keys
You need to have 3 pieces of data (Address, Secret Spendkey and Secret Viewkey. You can copy them from the wallet - for example, on the GUI wallet under Settings > Seed & Keys) from your wallet.
Open the GUI wallet and, after language selection, select
'Restore wallet from keys or mnemonic seed', then 'Restore from keys' and enter the data.
Using CLI wallet:
./monero-wallet-cli --generate-from-keys wallet-name.abc
monero-wallet-cli.exe --generate-from-keys wallet-name.abc
Next, you'll be asked for the Address, the Spendkey, the
Viewkey, and finally the new password for the re-generated wallet.
You can find all the info about restoring wallet from keys at https://www.getmonero.org/resources/user-guides/restore_from_keys.html.
Restoring from .keys file
You can also use the
.keys file which holds the keys info and the password to decrypt it.
.keys file is, by default, located in
C:\Users\<user-name>\Documents\Monero\wallets\<wallet-name> under Windows and
~/Monero/<wallet-name> for Linux.
On the GUI wallet, after language selection, select 'Open a
wallet from file' and open the
.keys file. Doing that, the
restoring process will be started.
Using CLI wallet, type:
./monero-wallet-cli --wallet-file wallet-name.keys
monero-wallet-cli.exe --wallet-file wallet-name.keys
Restoring from 25-word mnemonic seed
You must hold the 25-word mnemonic seed that you saved when creating your old Monero wallet.
Retrieve your 25-word mnemonic seed and launch the Monero GUI wallet.
After the language selection, select 'Restore wallet from keys or
mnemonic seed' and then keep 'Restore from seed selected'.
Using CLI wallet:
After the command, you will be prompted to insert wallet file name, password and the 25-word mnemonic seed. You can also choose a specific blockchain height to restore from. After you have entered the 25-word mnemonic seed and have chosen your specific blockchain height,
monero-wallet-cli will generate the same public address and view key as your old wallet and begin the refresh process automatically.
You can find additional info on this process on the official guide page https://ww.getmonero.org/resources/user-guides/restore_account.html.
I accidentally lost the 25-word mnemonic seed. Are the wallet coins lost?
If you have lost the keys, the .keys file, and the 25-word mnemonic seed, the coins remain on the wallet, but you will be unable to access them.