If your print head crashes into the print bed during calibration, it is possibly due to the nozzle not being lower than the fan. Below are images depicting this.
​
​
​
​
Correct Nozzle Height
Nozzle height too low
If the nozzle height is too low, it could be the fan is falling out of its position. Simply push it back up and apply a small amount of super glue.
Or you can unscrew the nozzle a little bit in order to get the nozzle tip to the correct position