If you are using Bitcoin-Qt and you encrypted your wallet with a passphrase, it cannot be unlocked without the passphrase.
Now, if you can remember some of the passphrase, and if the parts you don't remember aren't very complicated, it *might* be possible for someone to create a program that tries every possible variation of the parts you don't remember until they stumble on the actual passphrase. The more you can remember about what your password definitely did (or did not) have, the better the chance that someone can help you crack it.
Example that could be solved:
I know my passphrase was my cat's name followed by a number. I know that the number was more than 1 digit and less than 4 digits, but I can't remember what the number was.
Example that could probably be solved:
I know my passphrase used only lowercase letters and nothing else. I know it was less than 6 letters long. I know it was a real English word.
Example that could not be solved:
I mashed my hands randomly on the keyboard until the passphrase was at least 15 characters long. I'm know it was less than 30 characters. I remember seeing some numbers, some capital letters, and some punctuation, but I don't remember how much of each.
Some things to try and remember about your password:
- minimum length
- maximum length
- types of characters that were (or were not) in the password
- specific characters that were (or were not) in the password
- any real words?
- any pattern?
- any repetition?
- any other details