Crown came loose two days before trays came in - help!

Not the best timing but all will be ok.  If the crown can simply be recemented for now, this is the best scenario.  I would aim to finish Invisalign then circle back and address the tooth.  However, if the dentist feels the crown wont stay on for treatment or if there is an underlying problem with the tooth, best to either put a good temporary crown on or redo the crown now so you can move through the Invisalign process without any more issues.

I would wait for the trays to arrive and go to the dentist with the first tray. He can then cement the old crown with the trays in place during cementation. It will all fit, have no worries and make sure you don’t loose the trays. It will all be fantastic and worth it when you are done.

I'm sorry to hear this happened!  It must have been very upsetting, especially after waiting so long to get started with your orthodontics!  You'll need to speak with your orthodontist about your specific situation as the cost of additional aligners is different depending on which company is making the aligners (Invisalign, ClearCorrect, etc) and which product your doctor is using (a limited system with only 10 trays vs "full" treatment with unlimited trays).  In a "full" Invisalign case, for example, the cost of any additional aligners is included in the lab fee the doctor pays upfront for the product.  So if you lose a crown or need a new filling, you can simply take a new impression or scan once your new crown/filling is in place and get a new setup.  This would be a good situation to have your dentist and orthodontist put their heads together and see what the best option is for you depending on the status of your tooth (Does it need any other treatment like a root canal or core build up? Does it have a good long term prognosis with a new crown?) and the orthodontic plan (Which aligner product is being used? Will losing this tooth change the treatment plan?).  Don't be afraid to ask all of these questions to your doctors as I'm sure they will be happy to work with you to find a good solution.  Best of luck to you!