On approach to Samedan Airport (SMV), the crew did not obtain three greens. The nosegear did not appear locked down. After a go around it was established from another aircraft that the nosegear was down. A normal approach was then carried out. Upon landing the nosegear retracted and the pilot lost directional control of the airplane. It turned left, departed the runway and collided with a snow bank. The landing gear was probably damaged by a foreign object upon retraction after takeoff from Paris. Maintenance was accomplished on the landing gear during the 50-hours inspection prior to the accident flight. A tool was possibly left inside the landing gear bay. CAUSES: The accident was due to the fact that after touchdown the nose landing gear retracted due to damage and thereby control of the airplane was lost, whereupon it collided with obstacles.