I agree it is strange that this problem would arise without anything being visibly changed. Ultimately, though, in my experience troubleshooting these sorts of problems, something will need to be changed or swapped to get it back to normal functionality. It is not unheard of for the dialer component of the panel to seemingly randomly stop functioning properly. I would follow STR’s suggestion and have a fire alarm contractor send test signals from your panel and see if they are being received by the monitoring station. The next step I would follow to test and/or resolve the situation is to have them attach an external slave dialer and attempt to reach your monitoring station using that. If that works, they can leave it installed permanently.
While you can do some troubleshooting yourself, I would advise having a professional do any service work. It’s almost never worth the liability, and for the same reason, I don’t do any fire alarm work on the side.
If the fire alarm company that you have been trying to use is not responsive, you are free to shop around. Silent Knight panels can be serviced by just about any fire alarm contractor. Just be certain, as always, that they are properly licensed for service of fire alarm systems. The only issue you may run into is if the level 4 programming password has been changed from default, the new company will need to try to get this from the old one, and some companies are more amenable to sharing this than others.