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I had to leave my acoustic guitar at home. I had decreased the tension in the strings completely before leaving home...it has been 5 months...did I make a mistake?

Will my guitar still sound okay when I play it again?

Really, they're designed to be left under tension all the time.

The opposing forces of the neck [& truss rod, if appropriate] and the strings keep the neck straight.

Removing the strings will over time cause the neck to pull itself back - so when you re-string & tune it back to pitch, it's quite possible the neck will be out of alignment & you will get fret buzz.

Hopefully, leaving it in tune for a while will pull it back to where it should be. If it's bad, you could over-tighten by maybe a semitone [don't go silly with it, less is more ;) & see if in a couple of weeks it's starting to return to normal.

I wouldn't be inclined to take any steps other than that initially. Leave the truss rod alone, just use string tension, otherwise you may have to serially re-adjust as balance is reached, then overshot, then reached...

There was no need to slacken the strings. But you're unlikely to have done any harm.

I have a guitar in storage that belonged to a friend who died 9 years ago, almost to the day. (RIP CF.) I opened the case the other day. It's still in tune.

It'll be fine. When you get back to it, put some new strings on and tune it - you shouldn't have any issues. Any minor issues can be fixed by paying for a professional setup on it. Really, it probably won't even need that.

I had a guitar in storage for ten years, brought it to the USA and restrung it - it had no problems whatsoever.