OK, spark or fuel ...
Pull one spark plug out, reconnect it's cable and rest it against the cylinder head so the end makes contact. Don't hold it in place if you can avoid it, but if you have to make sure to use insulated plyers, otherwise you are in for a shock! Turn the engine over for 2-3 seconds, you should see the plug sparking. If it doesn't check the other side. If still no spark, checking the CDI and alternator are the next steps (you should find threads here explaining how to do that).
If you are getting spark, you likely have a fuel problem. Typically that's a carb that's gummed up (needs a chemical soak, sonic, 100psi air blast), rust in the gas tank, blocked fuel lines. Again there should be threads explaining those procedures.
1974 CB125S (purchased 1983, ate 2 crankshafts, sold 1984)
1986 Rebel 250 / N. California (purchased 2019)