If one branch were somehow to usurp power from the others, it would be able to act in any way that it wanted. For example, if somehow the president were to gain the power to rule by decree, then he (or she) would be able to make any laws he wanted without having them rejected by Congress or rejected by the Supreme Court. If that president wanted to take away people’s rights, he could. If that president wanted to enact laws that were constitutional but were severely unpopular, that would be possible too. There would, in essence, be dictatorial government.
If Congress got all the power, things would be much different. The US government would then resemble the British system. In that system, Parliament has the great majority of the power and all of the members of the executive branch are part of Parliament as well. This would mean that whichever party controlled Congress would be able to run the government. For example, the Republican Party today would be able to enact all sorts of laws without having the president veto them. However, it would not be a dictatorial system because the Republican leadership would at least have to get majorities in each house in order to pass legislation.
Of course, neither of these scenarios is at all likely to occur since our Constitution prevents it. Instead, a given branch can only have a little more power than any other branch and that generally only lasts a while. For example, if a president is very popular, then he or she can tend to push Congress around a little bit. If the president is unpopular, the Congress can prevent them from having much influence. However, these are temporary conditions and do not have a major effect on our system.