You can say "en öl" when you would say "one beer" in English. You're talking about one unit (a glass) of beer.
However, when it is treated as an indefinite quantity, it becomes neuter.
In this case, we're talking about beer as something we can't count, not as one (or several) glasses of beer.
I hope this helped.
For this particular phrase though, it would be ok in most cases to say either of "det finns ingen/inget/inga öl". For coffee/tee, only "inget" sounds right.
People ask that sometimes, I guess it depends on how precise one wants to be, but in reality ale is actually ale in Swedish too.
How would I write the sentence - It was not beer ? Would it be- Det var inte öl ? But then maybe I've not completely understood the difference between var and fanns in this context.
You've got it. It's the same difference with är and finns. Remember that while "there" can refer to a location in English, it can also serve as in impersonal pronoun, like "it". "There" is especially used in existential sentences, like this one, in the same contexts where Swedish uses finns/fanns.
It was not beer: det var inte öl. There was no beer: det fanns inte öl. It is not beer: det är inte öl. There is no beer: det finns inte öl.
finna vs finnas? I see that fanns is passive and means "there was", so what would fann (active) translate into English as? :)