In the unfortunate case your bike is gone, two things might have happened: either the municipality has taken your bike or your bike has been stolen.
If your bike is gone first find out if the municipality has picked it up. You can do so by using the search function on lostandfound or by visiting the bike storage depot of Maastricht Mondays between 13.00 and 17.00 or Thursdays between 15.30 and 16.30. The depot is located at Sint Gerardusweg 39. Try to bring as much proof as possible that you own the bike. This can be bike keys, a receipt or sales agreement, or photos of your bike.
If the municipality has not picked up your bike or the circumstances are suspicious, always file a report at the police station or online. Try to gather as much proof as possible that you own the bike. Unfortunately the rate of stolen bikes returned to their true owners is quite low at the moment.
In case you witnessed a bike theft you can file a report with the police online. This page provides more info about bike theft. You can also check this Facebook group for more information and discussions about bike theft in and around Maastricht.