He's been missing for quite some days now and whenever I try to hire someone else, he/she says I have to give up my previous comrade to join me. I don't know where and how to find him!!

All I can say is to wait awhile, like 4 to 7 days until the show up or they go back to where you recruited them. Sometimes I tell my companions to wait somewhere then I forget about them but after a few days they will go home. Hope this helps

If you can't hire another follower then there's good news. It means he/she isn't dead.