DIAGNOSIS Over time, grease and debris can get into the hinge mechanism, gumming it up. Also, the hinge may just be mechanically tight from the factory or after reassembly.
FIX Unplug and cool the grill. Work the hinge open and closed repeatedly. Sometimes this can loosen it up. If it's gummy, wipe any visible grease from the hinge pins with a paper towel and a drop of mild dish soap, then wipe dry. You can apply a tiny, tiny amount of food-safe machine oil (like mineral oil) to the hinge pins with a cotton swab, but avoid getting any on the plastic housing.
IF THAT DIDN'T WORK Check if the grill plates are installed correctly. If they are misaligned or warped, they can create friction against each other when closing, making the hinge feel stiff. Remove the plates and see if the hinge moves freely without them.
HEADS UP Don't force it. If the hinge is truly seized, forcing it can crack the plastic housing or damage the hinge mechanism itself, which isn't a separate replaceable part. You'd likely need a new grill. Always support the lid with your hand when opening, don't just let it snap back.