"Invalid mnemonic" or an empty restored wallet usually means a format, order or seed-type problem rather than lost funds.
Why it happens
BIP39 phrases use a 2048-word list and a checksum, so typos and swapped words are rejected. Electrum creates its own seed type by default, which BIP39 wallets do not accept.
What to check
- Work offline in the official wallet only; never type a seed into a website.
- Compare every word with the BIP39 word list and check the word count and order.
- Restore Electrum seeds in Electrum.
- If the phrase is accepted but the balance is zero, check the passphrase (the 25th word), address type and derivation path.
Next time
Write down the wallet app, seed type and whether a passphrase is used, and test the backup before funding the wallet. A swap such as BTC → USDT needs only a receiving address, never your seed.
Never "recover" a word online
Any site that receives your seed controls your funds. The BIP39 list is public and safe to read, but never type your phrase into a browser. If the seed may have leaked, create a new wallet after regaining access and move the funds.
Checklist: every word is on the BIP39 list, there are 12, 18 or 24 of them in the recorded order, you use the same app as at creation, and the passphrase is entered if one was set. If all of that matches but addresses differ, check the address type and derivation path before assuming the funds are gone.
FAQ
Why does my wallet say invalid mnemonic?
A typo, a word outside the list, wrong order, or an Electrum seed in a BIP39 wallet.
The phrase works but the wallet is empty — are funds gone?
Usually not; check the passphrase, address type and derivation path.