I bought potting soil (good stuff) a year ago. Since then the bag dried out.
What can I do to get it moist again? Should I just water it?
Just add water. Leave it in the bag and poke a hose right into the plastic and fill. Poke into another spot and fill. Leave it to absorb the moisture. Soil and other rooting media if allowed to get too dry becomes hydrophobic; the soil will shed water as if the soil were covered in oil.
Potting soil is sterilized. That is the beauty of potting soil. Safe for plants and animals. Definitions are all over the board. You need to read that label, let us know what it says.
Sterilized potting soil that has had a rupture of the plastic bag would easily have been found by opportunistic life forms but that would have had to happen before the bag of 'soil' dehydrated.
Just add water, allow the media to absorb the water overnight. Poke the business end of the hose into the bag puncturing the plastic and let it fill. Pull the hose out and punch into another area and continue to fill. Leave it alone for a day. Slice open garbage bags and then dump the bag of soil on the plastic (or newspaper) spread it out to drain and dry out a bit. Then use in your pots.
Don't use anything else in your pots other than that soil. And a bit of balanced fertilizer.
You should be fine.