A big player in the solar-energy installation market has closed, according to PSEG Long Island and several former employees. Level Solar, which operated offices in Hicksville and Ronkonkoma, terminated employees Tuesday, according to a copy of a note sent to one of the employees. The company’s website has been suspended, and calls to the company’s Long Island and Manhattan offices were not answered. In 2015, the company reached an agreement to borrow up to $25 million from the state’s Green Bank program.