You could check your emails. They often send out emails containing your wallet backup.

It's possible your wallet may have become corrupted somehow, or if you were storing that password in notepad, you might have edited it by mistake and saved it.

Maybe the most important thing to realise is that there are two blockchain.info's now. There's the legacy wallet, and the new (beta) wallet. If you migrated over, you use the new beta wallet, if you haven't, you use the legacy login.