You go to move the car and a wheel will not turn, or it drags so hard the car barely rolls. When a brake locks a wheel, a few specific culprits are usually behind it. Here is how to figure out why.
A seized caliper
The most common cause is a seized brake caliper - a rust-stuck piston or frozen slide pins that keep the pad clamped hard against the rotor; see caliper won't retract and how to unstick a seized caliper. That corner will not spin freely and gets very hot if you do manage to drive.
A frozen or stuck parking brake
If a rear wheel will not turn, the parking brake may be seized - especially after sitting in damp or freezing weather; see parking brake frozen in winter. Rust can bond the shoes to the drum or a corroded cable can hold the brake applied even after you release the lever.
Rust after long storage
A car parked for a long time can rust the pads to the rotor or the shoes to the drum, gluing the wheel in place. Gentle rocking in drive and reverse often breaks it free with a clunk - see brake noise after sitting. This is common on vehicles that sat all winter.
What to do
Do not force the car hard - you can damage a cable or hub. Diagnose whether it is the caliper, the parking brake, or rust, and free or repair the cause. A dragging brake overheats and wears fast, so fix it before driving, then fit fresh pads and hardware.