Query on sudden apperance of silt in a running borewell

What could be the reason and remedy?


I got a bore dug in March 2014. At 18  feet we got the rock surface and water at around 350ft, and the digging was stopped at 565 ft. We had put 7.5 hp 12stage 3phase motor pump at 400ft. The water output is around 4500 ltr per hour.

However since the last two days sand particles are coming out and blocking the drip filter. What has happened to the bore and what should I do now?