In addition to ice, lockup could also be the transmission is caught in two gears at once. It happens a lot when the shift lever gets worn.
Can you turn the engine over with the shift lever in neutral and the clutch engaged? If not, you've found the problem. To correct remove the lever, then use a big screwdriver or tire iron thru the shift lever opening to recenter all three rails. Then replace the shift lever - the transmission should now be free.