You're going to have to work through and eliminate the possible causes.
1. Is your coolant level correct?
2. Is the drive belt to your water pump slipping?
3. Is the radiator matrix blocked?
4. Is the thermostat stuck closed?
5. Is there a collapsed hose in the system?
If the fault has got worse gradually it may be the cooling system needs flushing through to remove sediment that can restrict the flow. If it has come on suddenly it is more likely to be the thermostat. Other possibilities are to do with the engine producing more heat than usual, but you would have other symptoms if that were the case. If you can take the thermostat out and flush the system through and then fit a new thermostat that deals with a number of potential causes in one go. Good luck
Richard
December 2008