I removed this screw from toggle switch and can't put back in. It just won't go back in. Is it possible to put it back or I was not supposed to take it out completely?
The screw is given a burr to discourage removing it completely. This can mess up the threads when it is removed. You can recover the threads with a tap and die, but for the cost of a new SPST switch it's probably not worth the risk. That said, often the threads aren't too badly damaged and you can put it back together by filing the tip of the screw and then being careful to screw it in straight.
Honestly, just replace it. Switches don't last forever and the cost of a new one is negligible. I'd buy one of the nicer "side stab" (Leviton calls these "preferred" and the side stab part "side wiring") where you put the wire under a metal plate and tighten it (i.e. no shepherd's hook). These are less than $2 brand new. And your switch won't look yellowed anymore.