My beer has started smell like cider on second day. Temperature is fine and hydrometer readings normal.it looks ok too. Nothing suspicious other than it now has no head. Today is now the 6th day in its fermenter. Please help
Cider smell can be caused by Acetaldehyde which can cause apple smells and flavours.
As the beer hasn't been in the fermenter long, you will notice this smell will decrease as fermentation continues, and afterwards, when the yeast will continue to "clean-up" these compounds.
You're fine, no need to panic. Leave your beer alone for another 2-3 weeks. Seriously, don't touch it, look at it, think about it, etc. Just leave it be for as long as you can stand it, and bottle it after 2-3 have passed.
Regarding the smell, fermenting beer throws off all kinds of crazy, nasty, wonderful, weird smells as part of the fermentation process. Its not uncommon to get sulfur fumes (rotten eggs) in some beers, which won't have that flavor in the finished product. Any kind of sharp, fruity, spicy smells are perfectly normal. As long as it doesn't smell like wet garbage or excrement, you're probably ok!
Regarding the lack of head, this is perfectly normal too, and that "head" you saw on the first couple days wasn't the true "head" that forms when you pour the beer regardless. What you saw is referred to by the German(?) term "krausen", and its just the foamy product of the fermentation. It can come and go fast, sometimes it sticks around til bottling, but usually drops back into the beer within a couple days, leaving a much smaller layer of white/brown splotchy film on the top of the beer.