What has likely happened is that the cables are rusty on the inside and won’t retract back through the sheath when you release the handle. As the cables are not retracting now when you pull up the handle it actually isn’t pulling back the cables against the caliper mechanisms. The cable holding in the pulled up position means the caliper is being held in the locked position.
There are springs on the cables near the ends where they interact with the caliper emergency brake lever on the caliper itself.
Alternatively, the mechanism on the caliper is frozen and won’t allow the cable springs to pull the lever arm back and as a result the rear brakes are locked.
What can you do?
First jack up the car, remove a rear wheel, ensure the car is on a jack stand and you will see the caliper in front of you. Next locate the lever on the caliper and try to move the emergency brake mechanism so the lever moves towards the cable. Often this will force the cable back and allow the caliper to release. Check this by trying to rotate the disk to verify it has released. If moving the lever towards the cable disengages it from the lever, the cable is likely frozen and will need to be replaced. If the cable moves freely back into the sheath you may need to look into rebuilding the rear calipers.