Learn Icelandic: Tölvan klikkaði aftur í gær. - The computer failed again yesterday.

You are right that klikka means "to click" (like with a computer mouse), but in everyday spoken Icelandic it is heavily used as slang to mean "to fail", "to malfunction", or "to mess up". When talking about a machine or a plan, it means something went wrong.

Why is there a preposition in front of the word for yesterday (í gær)?

Unlike English, which uses "yesterday" as a standalone time adverb, Icelandic requires the preposition í. The word gær is an old noun that is almost never used on its own in modern Icelandic, so you must always use it as the fixed phrase í gær.

How would the sentence structure change if I started with í gær to emphasize the time?

If you move í gær to the beginning of the sentence, you must apply the verb-second (V2) rule. The verb klikkaði has to stay in the second logical position, which pushes the subject tölvan into the third spot. The sentence would become Í gær klikkaði tölvan aftur.

I sometimes see aftur translated as "back". How do I know when it means "again" like it does here?

Context usually makes it clear. When combined with verbs of motion, such as að koma (to come) or að fara (to go), aftur normally translates to "back". With non-motion verbs or action verbs like klikkaði, it simply means "again", indicating a repeated event.