Hi, i am noticing a change in my thumb nail since 1month, it has become beaded, flaky & today i see a black spot at bottom. what could be the reason?

Sounds like a fungus: infection, unless you have had an injury which you don't mention. Sometimes injuries are followed by fungus infections. If the black spot enlarges or it becomes painful, you should be seen. If not, it is usually not dangerous though unsightly. Certain lasers can cure such infections. Most topical treatments are not effective and oral treatments also not too effective and have side effects.