My friend feel on my big toe now it's yellowish. What will happen? Will my toe nail grow back on it's on without me pulling it off?

Trauma. Can induce a thick or fungal toenail. You may want to visit a podiatrist for them to determine if the yellow discoloration is fungus or not. If it is you would want to treat that so it goes away and does not get worse.
Yes. Usually nail trauma causes blood under the nail. If it is not painful then just leave it alone and it will grow out over a 6 month period.