Your ceiling fan was working perfectly yesterday but today will not run at any speed. There are a number of potential reasons for this problem, including mechanical as well as electrical. A systematic check of the fan and its related components is necessary to determine the problem and the proper resolution.
Check the electrical supply to the fan. Check the circuit breaker or fuse that controls the power to the ceiling fan. It is possible the breaker tripped and the fan is simply not getting power. If the breaker is tripped, reset it and monitor the fan for a period of time. If the breaker trips again it is an indication of a short somewhere in that circuit.
Check the electrical power at the wall switch. Remove the face plate from the switch so you can see the interior of the switch box. Use the circuit tester check that power is getting to the switch and that the switch is working properly. If the switch is not working, replace it.
Remove the housing cover on the ceiling fan to expose the electrical connections. Check all connections to make certain no wires have come loose and are no longer making contact. Use your circuit tester to make certain the fan motor is receiving electrical power. If no power is reaching the motor, the problem is in the wiring between the switch and the fan.
Examine the fan's motor shaft. Check that nothing has become entangled or wrapped around the shaft which is preventing it from turning. Remove any obstacles or foreign objects from the shaft or motor, and attempt to activate the motor. If there is power to the motor and no obstructions, it is likely the motor has failed, and you will need to replace it.