We have an official page on troubleshooting the homing switches here:
As @neilferreri noted, Grbl error 9 is:
9 G-code lock G-code commands are locked out during alarm or jog state.
It might be that the machine is starting up on a switch, or a switch is (hopefully temporarily) stuck closed, or faulty. Let us know what you find out and if you need anything, contact us at support@carbide3d.com