If a person has a halachic שאלה and is in a situation where he cannot ask or find out the halacha (for example, he must decide what to do immediately, traveling with a dead phone), what should he do ?
Some General Guidelines:
Safek pekuach nefesh - if the situation is possibly life-threatening, one should act to save a life.
Safek d'orayta l'chumrah - if it's a doubt about a d'oraysa, he should be stringent.
Safek Brachot l'hakel - if it's a doubt about saying a blessing (of Rabbinic origin), don't say it.
Shev v'al ta'aseh adif - if you're in a situation where you may transgress a prohibition (of equal severity) regardless of which course of action you choose, choose the passive action rather than risk actively violating a prohibition.