Remove the wheels, the spring loaded pawls are likely full of crud and won't move freely.

Right - clean and grease the axle-end gears which contain a ratchet. The drive axle should have a greasing hole in the end, but you should clean that before forcing the dirt into the ratchet. Using a grease gun, shoot #2 White Lithium grease in until it comes out clean. Clean and grease the wheel/axle bolt and ring gear. This is annual maintenance along with other items in your manual.
In some cases, the problem is lack of adequate free play on the drive control, resulting in the belt being too tight, but since your mower had been working fine, it's most likely a question of lubrication maintenance.
Another version has a clutch housing/wheel adjuster at each rear wheel with a grease fitting on it. With these, you must not put too much grease in - one pump per year is adequate. If you find no grease hole in the end of the axle, clean/lube the gear assembly on each side with spray White Lithium grease. The solvent cleans, then evaporates, leaving the grease.