Invisibility doesn't affect the cover rules (and neither does (not) knowing the creature is there)

Anything that is an obstacle can provide cover for a creature. All that matters for the sake of the rules is that that thing can physically interrupt the path of whatever is trying to cross through it. A creature, invisible or not, can provide cover against spells (which require an uninterrupted line from the source to the target).

Normally, a creature counts as half cover:

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be [...] a creature, whether that creature is an enemy or a friend.

If the creature is large enough a DM might rule that they provide Three-Quarters cover or even Total cover, but the default for a creature is half cover.

Let's assume that the invisible creature gives half cover in your example. That means that if you cast fire bolt with an invisible creature between you, the target creature (not the invisible one) will get a +2 bonus to its AC, making it that much harder for your spell to hit it.

Unless the creature is providing full cover to the intended target (see next section) nothing about spell targeting or line of sight is changed.

The fact that you do or do not know about the creature has no bearing on the matter. Your DM will know and will adjust the results accordingly.

By the default rules, the creature providing cover does not get hit in any case.

Special case: full cover

If your DM rules that the creature provides full cover against the target, then you will not be able to cast your spell. To target something with a spell you must have a clear path:

To target something, you must have a clear path to it, so it can't be behind total cover.

So, in the rare case that the creature in your example is providing full cover to the intended target (meaning it is covering them entirely), your fire bolt will simply fail to cast.

If your DM is using the optional rule for invalid spell target from Xanathar's Guide to Everything, they may rule that attempting to cast this spell would still burn the spell slot.

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended.

Otherwise, the rules are unclear if it would or if you simply would be unable to cast it.

Optional rule: hitting cover

There is an optional rule that your table can use that simulates the chance to hit the covering creature.

If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.