If you didn't back up your 12 English words seed phrase in paper, along with any manually imported private key or JSON file, you may still be able to regain access to your wallet funds by extracting the Vault Data that is stored by the browser in your computer.
You can read how to copy your Vault Data from Chrome and Brave browser here. If you are using Firefox or any other browser, steps might be different depending on how every browser stores the extension's data locally
In Chrome based browsers (Chrome, Chrome Canary, Brave) you can also use the Web Inspector's Console tab to type this command, which will show you the {"data":"gewun.......opQ==","iv":"JfVS.....==","salt":"jx4......FI="} part of the Vault directly.
chrome.storage.local.get('data', result => {
var vault = result.data.KeyringController.vault
console.log(vault)
})
Once you have the vault data (which comes JSON formatted) you can proceed to the Vault Decryptor that you can find in the link below and also see the code at GitHub.
https://metamask.github.io/vault-decryptor/
In the Vault Decryptor page you'll paste the {"data":"gewun.......opQ==","iv":"JfVS.....==","salt":"jx4......FI="} part of the vault data and the password you set for your wallet in the MetaMask extension. If successful, you'll see the result below the Decrypt button showing the "mnemonic" 12 English words seed phrase, along with any other imported "Simple Key Pair" (private key).
You can now use the seed phrase (and private keys) to restore your MetaMask wallet.
We urge you strongly to make sure that you always backup your seed phrase, and any manually imported private keys, so this never happens again.
What if you no longer have access to the Chrome browser but still have access to the files?
If your computer has crashed and you no longer are able to use Chrome browser or if you've been able to recover your computer files using some backup software, you may still be able to get the encrypted Vault from that backup.
If you were using the Google Chrome browser in Windows 10, you should be able to find the location of the Vault by going to this folder location (you need to be able to see the hidden files):
C:\Users\USER_NAME\AppData\Local\Google\Chrome\User Data\Default\Local Extension Settings\nkbihfbeogaeaoehlefnkodbefgpgknn
In that folder you'll see a file called 000003 (or something like that), open that file with a text editor or code editor software like Atom (https://atom.io/).
In the text editor search for the word "vault" and copy this part that looks like the one pasted below:
{"vault":"{\"data\":\"wwpXXtFCqZkYsWfeEwItZjJ0Cc7mRVjG47Dqh+ztL1PiCG6Izhg+zG0mM+H2ykyjz3X0RNhAE6IVsWFZamcZ47B4sVi4SvUxrMhARm5L3yHPxr3UsyGrOXmthyVMgEGmjwlmnFCNd2nMZ2o8/sRMra8FupurqevnBv57FiYpEEs7gPpFHv6587aL44MmKD8Snv4JLFqiqmlK82Waq5F+Iv9mw2sFVAL9mgZBSgFgbWdB3TsKVB2k\",\"iv\":\"rkUQlNcGTxBE0My7a/bCXw==\",\"salt\":\"HcKyNfGzaRALRQ0DlKgcIe5Uk30iI/M//oG6w8vX8Nk=\"}"}
Remove the two parts in bold. Now use the Search and Replace feature of the editor to change all the \" to "
You would now have something like this:
{"data":"wwpXXtFCqZkYsWfeEwItZjJ0Cc7mRVjG47Dqh+ztL1PiCG6Izhg+zG0mM+H2ykyjz3X0RNhAE6IVsWFZamcZ47B4sVi4SvUxrMhARm5L3yHPxr3UsyGrOXmthyVMgEGmjwlmnFCNd2nMZ2o8/sRMra8FupurqevnBv57FiYpEEs7gPpFHv6587aL44MmKD8Snv4JLFqiqmlK82Waq5F+Iv9mw2sFVAL9mgZBSgFgbWdB3TsKVB2k","iv":"rkUQlNcGTxBE0My7a/bCXw==","salt":"HcKyNfGzaRALRQ0DlKgcIe5Uk30iI/M//oG6w8vX8Nk="}
You can now go to the Vault Decryptor https://metamask.github.io/vault-decryptor/ and paste that in the textfield there, enter the password you set for the extension when you created your MetaMask wallet. You should be able to see your seed phrase and any manually imported private key below if you click the Decrypt button.
For other OS or browser these steps might differ a bit.