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.
When you log in, the system wants to know if it's a valid account or whether it should kick you back to the login screen.
According to Mike Sego, a former Gmail engineer, "shva" is an acronym for "should have valid authentication". Apparently, the parameter is only included after a successful authentication.
The 1 is the default value applied to the parameter check. It's also a shorthand way for programmers to say true, like when you have successfully logged in.
The other part, #inbox, tells Gmail to load up your inbox as the first screen. You can change that to one of the other folders (or even labels you've created) to load them up.
E.g., https://mail.google.com/mail/?shva=1#sent will show your Sent folder items.
https://mail.google.com/mail/?shva=1#label/narwhals will load up your "narwhals" label.
Gmail, like many web services, serves a standard interface that will change to show only your information and data when you've logged in.
The particulars are referenced on their end through the use of an ID from the cookies or sessions generated after the login screen.
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