I lost part of a large filling a few weeks ago. I tried my dentist only to be given the same advice as posters above. I ended up buying a filling kit online and tried DIY.
It was very tricky, the filling material set as I was mixing it and trying to get it into a back tooth required more than one pair of hands. The filling gradually broke up but lasted two weeks after which I replaced it again (this time it was even more difficult to put in so I have a partial filling).
During this time, I was contacted by my specialist re shielding, so I asked about dentistry. As my immune system doesn't work, it was suggested that I contact my GP to get a standby prescription in case I have a dental infection. So far I haven't but at least I know what to do.
What worries me is that it will take some time for dentists to get up and working again, my shielding lockdown is going to go on a lot longer. By the time I can make an appointment, it will be at the end of a very long list. I would have thought that dentists would have had some sort of triage system for when they do start again - I guess that is too much to hope for!