Hello! I am afraid that the family tree is lost, but definetly contact Support via "Send Feedback" (this is where people who have lost access to their accounts can restore them) and explain the situation - as far as I know, Support is generally very supportive and sincerely tries helping.
Depending on how badly you want the tree restored, another solution might be trying to recover your browser's backup files and basically "return" to the state your browser was bevor the glitch/reopening. I used this to restore tabs after a glitch rendered dozens of tabs with important info/research broken/blank. This however means deleting/removing everything you have done on your browser since the crash, and I can give absolutely no guarantee that this helps restore the cookies (or whereever family tree info for unregistered users is saved).
I sincerely hope you find a way to restore your family tree. For the future: besides working with an account, saving the family tree on your hard drive as a backup is also a good idea.
Best wishes!
