I've been using mist 7-4, windows, but it does the same thing in other versions. my crowdsale password has been working fine til today. other passwords from other accounts still work fine. what could cause this to happen?
One general thing to try, which isn't specifically related to Ethereum, but added here in case it helps anyone in future from going into meltdown mode...
If your password contains non-standard characters which are in different positions on different keyboard layouts, and if your keyboard has for some reason jumped between layouts, then you'll need to check you're typing what you think you are.
I use a UK-layout keyboard which has in the past jumped to US-layout for whatever reason. This can cause problems with characters such as
@ which are reversed in the two layouts...
(Summary: Type your plaintext password into a text file. Check it looks okay. Delete the text file.)
(Edit: I don't have a machine running Mist with me, so can't remember if it displays the passwords in plaintext as you type it. Or gives you the option to view it. My answer assumes it doesn't.)