First, as others have said, if the shed and fence were there when you bought the house, then it's on the HOA to say something to the former owners if they're illegal. You get a packet from the HOA assuring you everything is copacetic. What might help your cause, however, if is you happen to have any photographs when you purchased the house showing the fence and shed, or perhaps any documentation from the sale that notes the fence and shed somewhere. Anything that can prove these were present.

Second, what state are you in? And how old is the HOA? In many states, unless the HOA's covenants *specifically* provide the board the right to fine you for various violations, they are not able to fine you. So you may want to check the language in the covenants and see what's actually allowable. Otherwise they're just trying to scare you.

If it's indeed true that your fence and shed meet community standards, you should be able to appear at a meeting and simply tell them that you purchased the house with them in place, making them automatically approved, even if the previous owner did not. Most places will take your word for it, but sounds like you'd be well off to find that proof somewhere.