My cat aka my best friend has dissappeared, help!?

ok so since sunday morning , my cat bandit has been missing. its unlike him to do this, he comes home every day multiple times, in fact he is usually always begging to come in! since last week though, theres been a neighbor cat whos been following him around, and it cries for him whenever hes inside our house. could this be a female wanting to mate? she hasnt been meowing ever since hes dissappeared, is he off mating with her? plz help, i love my cat to death and im so sad hes gone, thanks everyone