Sounds like it's stuck in either 1st or 2nd gear. Shift fork may be broken or the end of the shifter that runs in the shift rails is worn, and has slipped over to the other shift rails. Either way it looks like the shifter will have to come apart on the transmission.
What transmission is it?
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!