Is that situation occured suddenly or gradually ?
You can remove the shifter lever by removing the 2 nuts close to the lever and lift the lever assy. There is a pin that hold the lever in the socket it may be worn or broken or may be the end of the lever is worn and jumps over the shifting rails and cause your problem. If it is the pin it is easy to replace. If it is the worn lever end it may be welded and milled to original shape.
Other than what stated above the shift forks or rails may be damaged or bent.