Just read my old Yelp post and wanted to update.
Allegiant did nothing for me and my situation. NOTHING...ZIP...NADA...ZILCH!
I was informed I could wait 2 days for their next flight out (and hope their ONE aircraft was going to be up and running to get me back to Phoenix) and buy a ticket at FULL PRICE! The customer service rep said I forfeited my ticket when I missed my flight so I would have to purchase another ticket if I wanted to fly back to Phoenix. A replacement ticket would have been about $300 one way.
I decided NOT to take my chances of getting back to Phoenix. I was already missing one day of work because of this fiasco. I ended up buying a 1 way ticket through Delta for $600 one way. But at least I got back to Phoenix that same day and was able to return to work the next day. $600 was hard to swallow but I would have missed 2 more days of work (for a total of 3 or maybe more) if I would have waited for another Allegiant flight.
To add insult to injury, Allegiant took my bag back to Phoenix on the flight I missed. I didn't think they were allowed to take baggage for someone that wasn't on their flight. So...after I was picked up from Sky Harbor in the afternoon, I went home, picked up my car then drove out to Mesa to pick up my suitcase (during rush hour no less). That added another 2 hours to my day of travel after little sleep from the night before.
Allegiant is STILL the WORST airline I have ever had the displeasure of doing business with. I continue to hear bad experiences about them and I am sure they will be out of business soon.