I haven't had access to physical books since university libraries have been closed. How do I address this in interviews, and will people dismiss my file because of this?
You need to be prepared to offer a lot more detail. Most academics have no need for entering a library anymore, so if you do, you need to explain that. If the library offered curbside pickup for some books, you need to explain what that did not work in your case.
Perhaps you work exclusively with rare documents and had medical reasons to not be able to enter a library for 18 months. Did you have other projects you worked on that did not require rare documents? Was your life so disrupted that you could not possibly take on new tasks?
By the way, adding more detail to your question is optional. What I am saying is you need to give more details during your interview or you probably will be dismissed.
Let me suggest two reasons for loss of productivity that have nothing to do with libraries.
The first is burn-out. If you went through an intense period of you life (academic or other wise) prior to the time the pandemic hit, you might just be suffering from that. It can be debilitating. My solution (long ago) for dealing with it was to make a rather large change. That brought me back and I was able to continue with renewed energy. A period of rest (that the burn-out itself generated) may have also contributed.
The second possibility is more serious: depression, which is a medical condition. I don't know much about it, but do know that one should talk to a professional about it, probably a doctor. Clinical depression can lead to a lot of bad things beyond lack of productivity. Professional advice, and maybe medication, can help.
But make sure that you diagnose the issue accurately (or have it done by a professional), no matter what it is. Continuing as you are isn't likely to help.
You don't give a lot of background details, so let me also say that it doesn't seem clear why you would need to "address this in interviews" at all. Focussing on things you weren't able to do, and making excuses — even if they might be valid excuses — is usually not something that should be discussed in any great detail, unless there are some very good reasons (your area of research needs access to rare physical documents that are not available digitally, see the answer by Terry Loring). Try to focus instead on things you did do.
If you are asked specifically why you were not as productive as before, then answering that libraries were closed is usually not going to be helpful. However, showing that you reflected the fact that it took you some time to adapt to the new situation and find a new workflow is reasonable to admit openly. Yet again, the main focus of any interview should be your ideas and what you did achieve, not how external circumstances prevented you from doing what you wanted to do.