Depends what you mean by "lose".
If they get crushed as the early polls predicted, then the party will die. It has no other leadership ideas, no genuinely talented people in the party who are not discredited, and its membership numbers will collapse. the party will die and we'll be stuck with Tory rule until I am an old man.
If they don't form a government but are not seriously defeated (as the current polls predict), it will limp on for another election cycle and probably die after the next election.
If they actually win, then the MPs who VONC'd Corbyn will need to eat humble pie and submit to him. Party will continue to exist for the foreseeable future. Least likely scenario though.