The probability of a two-sided coin landing head side up is the same each time it is flipped. Since there are only two possible outcomes, heads or tails, there is a 50% probability that the flip will be heads. The fact that the coin landed head side up the first time it was flipped has no bearing on the outcome of the second flip.
However, this assumes that there are no forces that influence the coin toss; for example, freshly minted coins tend to be heavier on the heads side, so they tend to land head down more often than head up. To determine probability based on empirical evidence, one must continue to flip the coin, adding up the number of times it lands head up and then dividing by the total possible outcomes. For example, if a coin is flipped ten times and lands head up eight times, then the probability is 8/10 or 80%.