Sorry if I'm missing something, but again I don't know of a scenario that would lead to the issue you've described.
When you are creating an Electrum wallet (or later adding a password to an already created Electrum wallet without a password) there is a checkbox which says "Encrypt wallet file". This is automatically checked, so in the vast majority of cases adding a password also encrypts the wallet file and therefore prevents you from opening it without the password. You can, however, uncheck this box, which will then only password protect the private keys. So you can open the wallet to view the transactions and addresses, but cannot spend without the password.
i think i opened without a password the back up file, then its a watch only wallet because i can see all the transations but still cannot use the btc, i believe this btc is lost, thank u for your help
If the wallet is truly watch only, then it will say that at the top of the wallet window. Something like "Electrum 4.x.x - WALLET_NAME [watching only]".
If it is not watching only, and you have simply forgotten the password, then you could attempt to bruteforce it using
https://btcrecover.readthedocs.io/. This will require either for you to have some idea what the password might be, or to have used a weak password in the first place.