It would be a complete disaster for the Soviets. The Red Army in 1939 was not prepared for a major war for a variety of reasons:
1. The officer corps had been decimated by the Great Purge. 3 out of 5 Marshals, 15 out of 16 army commanders, 50 out of 57 corps commanders, and 154 out of 186 division commanders were executed. They were replaced by men who were often far less competent, and in any case were afraid to do anything that hadn't been approved by their superiors. The Red Army also had reinstated a system of political commissioners, who were equally as powerful as the officers who commanded units and whose job was basically to ensure that their fellow officers were loyal. Many experienced Soviet pilots and flight instructors were also killed, leaving an air force that was undertrained and unprepared.
2. The Red Army was working with poor equipment. The T-34s were not yet in service, and the tanks that did exist (the T-26 and BT-series tanks) were obsolete. German anti-tank artillery could easily penetrate their armor, and the T-26 design created major problems (for example when older T-26s were hit the rivets on the tank had a tendency to break off and fly at the crew like shrapnel). Maintenance standards were also extremely poor, which would have led to a lot of tanks breaking down.
3. The Red Army's logistics system was terrible. The kind of logistics structure needed for a major war simply didn't exist, and had the Soviets been forced to stay on the campaign for a long period of time their units would have suffered shortages in food, oil, and spare parts.
However it would also be a disaster for the Germans. Without the resources from the Molotov-Ribbentrop Pact Germany would have been unable to fight a major war for a long period of time, and at any rate they couldn't fight a two-front war.