Maybe another silly question: did you change some settings in your slicer in between? Normally the fan control is managed by the software.
Do you have an old .gcode file where you definitely be sure that the fan was working? You may test this printout.
Otherwise I would be lost with your statement “when I press start”.
However, you may check the hardware functionality using the menu structure:
A setting above 150 (max: 255) should definitely start the fans independent from any heating condition. It can be commanded with a terminal connection too:
M106 (optional: value like 150).