The Prophet (Sallallahu alaihi wa sallam) said, “The supplication of my brother Dhun Nun (Yunus, peace be upon him), who called on Allah while in the whale’s belly: ‘There is no deity but You. Glory be to You! Verily, I have been among the wrongdoers’ (Surahtul Anbiya 21:87) - no Muslim person says it, for any situation whatsoever, except that Allah Most High answers his call.” [Tirmidhi]
 
La ilaha illa Anta, Subhanaka, inni kuntu mina z-zalimin.
لا إلهَ إلا أنتَ سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظّالِمِيْنَ
 
 
Supplication for those whose affairs have become difficult
“O Allah! There is no ease except that which You make easy, and indeed You, when You want, make difficulties easy.” [Sahih Ibn Hibban]

Allahumma la sahla illa maa ja’altahu sahlan, wa Anta taj’alu l-hazna idha shi’ta sahla.
  
اللّهُمَّ لا سَهْلَ إلا ما جَعَلْتَهُ سَهْلا وَأنتَ تَجْعَلُ الحَزْنَ إذا شِئْتَ سَهْلا

Supplication for one in distress The Prophet (Sallallahu alaihi wa sallam) would say when in distress, “There is no deity but Allah, the Knowing, the Clement. There is no deity but Allah, Lord of the Magnificent Throne. There is no deity but Allah, Lord of the heavens and Lord of the earth and Lord of the Noble Throne.” [Sahih Bukhari]
La ilaha illa Allahu l-Alimu l-Halim. La ilaha illa Allahu Rabbu l-Arshi l-Azim. La ilaha illa Allahu Rabbu s-samawati wa Rabbu l-ardi wa Rabbu l-Arshi l-Karim.

لا إلهَ إلا اللهُ العَليمُ الحَليم لا إلهَ إلا اللهُ ربُّ العَرْشِ العَظِيْم لا إلهَ إلا اللهُ ربُّ السَّمَاوَاتِ وَرَبُّ الأَرْضِ ربُّ العَرْشِ الكَرِيْم
 
When the Prophet (Sallallahu alaihi wa sallam) would be afflicted with a stressful trial, he would say, “O Living and Eternal Maintainer! By Your mercy I seek help!” [Sunan Tirmidhi]

Ya Hayyu Ya Qayyum! Bi rahmatika astagheeth!
يَا حَيُّ يَا قَيُّوْمُ بِرَحْمَتِكَ أَسْتَغِيْث