The city council website's section on cars opens with the question 'do you really want to bring your car to Amsterdam'. Its a good question.
The website also has a good map of what parking costs - its a matter for the borough councils to decide.
If you get clamped, this is the number to ring: (020) 251 22 22 - they say its manned 24 hours a day. Someone from Stadstoezicht will come and unclamp you. You can pay with credit or pincards. If you want to pay cash, you have to go to a Pay and Go office. Ask where the nearest is when you ring. It will cost you at least €103.60. They only clamp in the city centre and are planning to phase it out altogether because it is so tourist-unfriendly.
If your car has disappeared, it might have been towed away. To find out, ring the council information number Antwoord 14 020 between 08.00 - 18.00 uur. If it is after 6pm, ring 020 251 2222.
If you have been towed away, your car will be on the Daniël Goedkoopstraat 7 which is open 24/7. Metro 53, 54 and fast tram 51, get out at Spaklerweg/OverAmstel.