Why do the bike wheels not turn when the gears are turned via the pedals?

If the bike is OK, then the rear wheel should turn when the pedals are turned in the right direction.

But a rear hub contains an one-way clutch, based (usually) on a set of pawls and a serrated ring.

If these pawls get gummed up in rust, dirt, dried up grease or something they won't engage. And if they don't engage, the wheel won't turn.

If you know where to aim, it's possible to flush the hub with a really light oil, which might restore function. Otherwise you'll have to have it replaced. It's a fairly fast and inexpensive job for a bike mechanic.