First thing that comes to mind is the brake interlock system. On the older machines, the shuttle lever can lock up if the brake pedal isn't fully pressed down.. give it a good firm press and see if that releases it. It's a common safety feature. If that's not it, the next thing I'd look at is the control valve for the transmission. Since it's been sitting, a spool could have gotten a bit sticky inside the valve block, especially if the fluid is a bit older. Letting it run and warm up for a few minutes sometimes helps to free things up. A problem with the linkage is also plausible like the others here mentioned.. Maybe a bit of rust or a stone could have gotten wedged in there while it was parked up. Don't force it, but check the connections under the cab.