You need fuel, spark, and compression. Check the compression and make sure you do not have a broken reed. Even if you have someone pull it in sixth gear and put your thumb over the spark plug hole. Be sure to ground the plug. Next check for spark. Pretty hard to see on a CR125. Ground the plug and cup your hands over it while someone spins the rear wheels. If you have that, then the box, stator, coil, and grounds are good. The last is fuel. Is the plug wet or dry? If its really wet, you also may have the bottom of the crankcase full of fuel. Ground the plug and spin the motor with a strap off of the wheels with the plug out. If you are flooded below, you should see a mist coming out of the plug hole. If so, put in a dry plug either new, or known good and let it go. You can also try turning it over on a start with the accelerator all the way down. If it catches, be sure to let up. CR125’s are really reliable and should not be hard to start. Check out some of these items and report back.