Most common causes are:
Dehydration is common among patients with diarrhea who lose large amounts of water in their stool, particularly when drowsiness
limits their drinking of fluids or is associated with nausea and vomiting. Dehydration also can occur with prolonged vomiting of any cause because of the loss of water in the vomitus. Other causes of dehydration include exercise, sweating, fever, and heat exhaustion
or heat stroke
. Individuals with mild dehydration may experience only thirst and dry mouth. Moderate to severe dehydration
may cause orthostatic hypotension
(manifest by light-headedness, dizziness or fainting upon standing). Protracted and severe dehydration can lead to shock, kidney failure, confusion, acidosis (too much acid in the blood), coma, and even death.
Removal/treatment of the cause.
Physical exercise daily.
High Protein diet.
As far salt is concerned take in moderate amount.
Yes you will regain normal BP if there is no cardiac disease.