What will happen if an F-16 flies higher than 50000 or 70000 feet?

Well, the f-16 isn't suited for extreme altitudes although it is designed for air-to-air combat. The service ceiling for the F-16 is 50,000 feet, so under standard conditions, any operation above this begins to affect performance such as lift and thrust.

If you're looking for an "it'll fall out of the sky" type answer, keep looking, and good luck. Military aircraft, especially a prime platform like the F-16, is largely classified. From personal experience with other classified weapons systems, that 50,000 foot service ceiling is low-balled. The unclassified operational ceiling of the U2 is around 72,000 feet, but it will fly higher.