I used to be able to sing falsetto until I had a very bad dose of full-blown influenza about 5 years ago.

As well as feeling like death, I also got conjunctivitis & laryngitis, so could scarcely speak never mind sing.

It took about 5 months before I was able to sing at all, but found the falsetto gone - small consolation though, I'd acquired a raspy 'blues' voice, so some singing style had to change to match this new 'gift'