Hello Shrivastava
Hydrouretonephrosis is mild collocion of fluid in kidney and upper ureter. This is due to obstruction like urethral stone in the urinary track. But the reason for the obstruction is still unknown as your sonography report does not shown any calculi.
Consult you urologist and you may need IVP or CT scan to find the cause.
As per treatment is concerned drink plenty of water, a course of antibiotic if you have burning maturation or fever. Can take some anti-spasmodic or analgesic for pain.
Rest of the treatment depends on the cause of obstruction.
Hope this will help you.
Best Wishes

Mild LT Hydroureteronephrosis means that there is mild swelling or dilatation of the urine collecting structures of the left sided kidney and left ureter. Its not a disease itself rather indicates that there is some obstruction to the flow of urine and that can be secondary to any stone in the urinary tract starting from kidney to the urethra, any fibrosis, in males due to prostate enlargement and in females pregnancy can cause this too.
You havent mentioned your gender.

Ultrasound, MRI, Cystoscopy, CT scan etc can be ordered to know about the cause depending on suspicion of the doctor.
The treatment is directed at the treatment of the cause. The blockade needs to be relieved either medically or surgically to prevent further damage to the kidney.

The prognosis of hydronephrosis is extremely variable, and depends on the condition leading to hydroureteronephrosis, whether one (unilateral) or both (bilateral) kidneys are affected, the pre-existing kidney function, the duration of hydroureteronephrosis (acute or chronic), and whether hydroureteronephrosis occurred in developing or mature kidneys.

For example, unilateral hydroureteronephrosis caused by an obstructing stone will likely resolve when the stone passes, and the likelihood of recovery is excellent. Alternately, severe bilateral hydrouerteronephrosis due chronic pyelonephritis and scarring fibrosis of kidney has poor prognosis.Certain points which would affect your outcome are if you have medical diseases like hypertension, diabetes, repeated urinary tract infections and history of pyelonephritis.
You should consult a urologist who will examine you and make a provisional diagnosis depending on which he will advise you ceratin tests to confirm the diagnosis. Also dont forget to get a urine for culture and microscopy done to rule out infection which is common in hydroureteronephrosis due to stagnation of urine. If present, antibiotics needed for the same.