Let us ponder over the story of Prophet Yunus (Jonah), peace be upon him, that highlights the power of sincere Du'a. When Prophet Yunus (Jonah) was swallowed by a whale, he had no chance of survival, logically speaking.  

By the way, the blue whale weighs 150 tons; 50 tons of flesh, 50 tons of fat, and 50 tons of bones. Also, a single blue whale could yield up to 90 barrels of oil, and its mouth is so huge that man can stand inside it. So, when Prophet Yunus (Jonah) was in the belly of the whale, he had no chance of survival.

Nevertheless, Allah the Almighty answered his call and delivered him from the distress. Allah the Almighty turns the survival story of Prophet Yunus into a general rule that is applied to every believer. He says:

"And thus We do deliver the believers (who believe in the Oneness of Allah, abstain from evil and work righteousness)"

Dear brothers, no matter how hard your situation is, it will never be  worse than the one of Prophet Yunus, in the whale's belly. Allah the Almighty says:

"But he cried through the darkness (saying): La ilaha illa Anta [none has the right to be worshipped but You (O Allah)], Glorified (and Exalted) are You [above all that (evil) they associate with You]. Truly, I have been of the wrong-doers. *"So We answered his call, and delivered him from the distress…"

[Al-Anbiya': 87-88]

Although this story is over, Allah,  Exalted and Sublime be He, turns it into a rule, He says:

"And thus We do deliver the believers (who believe in the Oneness of Allah, abstain from evil and work righteousness)"

[Al-Anbiya': 88]

Undoubtedly, Allah's Promise will be fulfilled everywhere and at all times. Thus, although there was no chance of survival, according to logic, in that extremely rare situation in the belly of a whale, Allah the Almighty answered the Du'a of Prophet Yunus, peace be upon him, and delivered him from the distress.

Dr. Umar Abdul Kafi:

Al-Hassan al-Basri, may Allah have mercy upon him, had full trust in Allah and believed deeply in the power of Du'a, so he used to say, "Whenever I make Du'a to Allah the Almighty, I believe with certainty that He is fulfilling my need at the very moment of making Du'a". Thus, the true believer should reach such a high degree of certainty. 

Dr. Rateb Al-Nabulsi:

The believer is in desperate need of Allah.

Actually, putting your trust in Allah the Almighty gives you a great sense of tranquility; you will deeply believe that Allah the Almighty loves you, and that He will never neglect your Du'a or let you down. That is how the situation of the true believer should be; he should always feel that he needs Allah the Almighty desperately.