Diagnosis
The plastic wheel hubs on these grills are notorious for seizing, especially if left outside. The axle rusts, the plastic warps, and they just lock solid.
How to Fix It
You'll likely need to replace them. The wheels are usually held on by a push-on retaining clip or a cotter pin. Remove the old wheels. Before installing new ones, clean any rust off the metal axles with sandpaper and apply a light coat of silicone lubricant or grease to the axle to prevent it from happening again. Char-Broil sells official replacement wheel kits.
If That Didn't Work
If the axle itself is so rusted it's damaged the mounting bracket, you might need a more involved repair. In a pinch, you can lift the front of the grill (with help) and slide a furniture dolly under the leg to move it.
Heads Up
When you get new wheels, consider slightly larger, all-terrain replacements if you need to move the grill over grass or gravel. The stock wheels are pretty basic. Keeping the grill under a cover slows, but doesn't always stop, this particular failure.