I was removing my mezuzas to get them checked after about four years, And I was shocked to find that one of the mezuzas cases was empty. I’m generally overly careful regarding matters like this, and I don’t know how I could have possibly put up an empty case. In any event, was I oveir a deoraisa for all this time? Is it considered a violation for each moment or one generally violation?
Although it is considered a violation for each moment, your case does not seem to be one of negligence. Therefore it is considered bi’onus (beyond your control), and you should just try to make sure that it doesn’t happen again.
All the best
Minchas Chinuch 423 says that not having a mezuza when necessary is a violation every single moment.