It's quite simple actually for a hacker to hack 98% of the population.
If your password is related to a credential of yours, something you would put on a hospital application or such, it is extremely easy for them to guess it. They don't even have to try every permutation of letters like they might have to. Hence
p3anu+01l5uck5 is not a good password.
They can also try to hack another account, and since 94% of the computer-literate use the same password everywhere, it's insanely easy for someone to get your password if they can hack a low-level forum site you use that password.
And those are just the easy ways. There are still more advanced techniques (phishing being the biggest one, but there's also brute-force/dictionary attacks, viruses/spyware, simply someone watching over your shoulder, or even as complex as blackmailing Google with even more account information) they can use.
But it's also possible that you forgot your password. It's hard to tell the difference nowadays.
And if this ever happens again (hopefully not), or anyone else, Google has a form for this purpose.