I think that the answer is a bit complicated (from a Christian point of view). There has been instances where Biblical characters have prayed and things have changed for the better. At times, prayer has ended up postponing disasters (Kings Ahab and Hezekiah).
In my opinion, there are two destinies for everyone.What kind of destiny will affect your life depends on the choices you make.
For example(Christian view point again), God may have meant for you to be... lets say a missionary/businessman/president etc in your lifetime.Unfortunately you make a choice to give up when the going gets tough and you commit suicide.See?The suicide was not part of your successful life but you chose to end it before life changed for the better.You lose your destiny and a replacement will be found.
History ends up being rewritten because you have lost your place in it.Your loved ones end up being affected negatively for the worst, things start going wrong for them.Alternatively someone ends up learning from that suicide and changes their lives for the better.History ends up being rewritten again for the better this time.
EX 2-Alfred Nobel was the creator of dynamite (i think); a chance obituary about his creation ends up causing him to change his life's course and he founded the Nobel prizes.Tell me how many people remember about the dynamite thing in relation to him?
I know i have rambled a bit but i wanted to explain about prayer and how it can change things.Prayer is a spiritual way of implementing things in the physical.
You need to have faith and the subject of your prayer needs to make a good choice.everyone has free will and will choose what they want to do regardless of what you pray for.A situation on the other hand, may probably change for the better if you pray and your heart is in the right place.God is God and He will make the final decision.
Sorry about the rambling again but i hope you have gotten something out of the whole thing.