But if you would recover wallet.dat file from old backup somewhere from time when you had no password set up than you could access all funds on main wallets even if you encrypted it afterwards. 
good luck.
Good idea about looking for a possible unencrypted backup. good luck.But take note that in that case; it's only useful if the unspent transactions outputs are associated with the old wallet.
The new keys that the wallet created after it's encrypted can't be recovered nor derived deterministically by the old unencrypted backup since after encryption,
it set a new HDseed (for HD wallet.dat) or set a new set of keys in its keypool (for non-HD wallet.dat).