My big toenail is falling off on the top half from onychomycosis. will it ever grow back?

Good probability: There is a good chance of the the nail growing back. If you treat the fungus while it is growing back there is a better chance of growing a healthy nail in place of the diseased one.

Likely yes: Might be wise to treat that nail with at bare minimum a topical antifungal agent. I do feel oral meds are much more effective than topicals. Leaving the toe alone will not help improve your condition, so if it bothers you have it treated.