It is not easy, there are a few different kinds of ways you can try to get the password.
Brute-Force is one of them but also dictionary attacks can help.
If you got a part of the password you may also try a Biham-Kocher attack.
You can google how to do all of these, it is not hard, but unlikely that you succeed in a short period of time if your password is strong enough.
Depending on how exactly you encrypted the file, there might be other, more complicated, ways to gain the data back as well. But a strong algorithm was used, there is no way without getting the password somehow.