Will my toenail grow back if i have severe toenail fungus?

Yes: They will likely come back but may be mildly thickened.

More detail: Are you asking can you ever get a normal nail if the fungus is severe? Yes, you can. You would have to treat it by either oral meds i.E lamisil, (terbinafine) laser therapy or possibly nail removal and then aplication of topical. Or any combination of the above. It won't happen by itself.