If you go strictly by this wording, the loss only persists until the negative level is removed and thus the spell would be available again.A spellcaster loses one spell slot of the highest level of spells she can cast and (if applicable) one prepared spell of that level; this loss persists until the negative level is removed.
Thanee said:[...]I'd probably not do it like that, however, rather let you regain the access to the spell slot, but count it as drained/used up for the day.