if you had done anything with fuel system, like new lines etc... it can happen that you get the pickup line connected to wrong carb port, it happened to me when i changed my lines out, was so puzzeled as to why it wouldnt fire up. then traced the lines from carb back into tank and discovered i had the inlet line on wrong carb port. swapped them and it fired right up.
also it may sound stupid, but have to ask, did you prime the carb with the bulb? pressing it repeatedly till fuel is to carb? did you choke the carb on first pull or two? take choke off it it tried to fire?
i know it sounds stupid to ask, but things like this do happen.
pull the plug stick it in the plug cap hold it to engine metal and pull rope look for spark. thats another check you can do .