THE PLT STATED THAT THE LANDING GEAR SWITCH WAS DOWN WHEN HE STARTED THA TAKEOFF ROLL. WHEN THE ACFT HAD ACCELERATED TO APRX 50 MPH DURING THE TAKEOFF ROLL, THE LANDING GEAR RETRACTED/COLLAPSED, NOSE WHEEL FIRST, FOLLOWED BY THE MAIN GEAR. THE PLT STATED THAT AFTER THE ACFT SLID TO A STOP, HE MOVED THE LANDING GEAR SWITCH FROM THE 'DOWN' TO THE 'UP' POSITION BEFORE HE DEPARTED THE ACFT. MAINTENANCE PERSONNEL WHO WERE AT THE ACFT IMMEDIATELY AFTER THE MISHAP, CONFIRMED THAT THE LANDING GEAR SWITCH WAS IN THE 'UP' POSITION WHEN THEY ARRIVED. AN INSPECTION OF THE LANDING GEAR & GEAR WARNING SYSTEMS REVEALED NO EVIDENCE OF A PRE-ACCIDENT MALFUNCTION. THE GEAR WARNING HORN SOUNDED WHEN CHECKED WITH THE SQUAT SWITCH OPEN. THE PLT STATED THAT HE DID NOT HEAR A HORN DURING THE TAKEOFF SEQUENCE. ALSO, THE PLT NOTED THAT THE FLAP SWITCH WAS IN THE 'DOWN' POSITION, WHEN HE ARRIVED AT THE ACFT, BUT THE FLAPS WERE IN THE RETRACTED POSITION. HE BELIEVED SOMEONE HAD PUT THE FLAP SWITCH DOWN AFTER THE MASTER SWITCH WAS OFF.