You didn't break anything — this is a known QuickBooks issue, just not a common one, which is why 3 years without seeing it makes sense.

What it actually is: QBCFMonitorService is a background Windows service that QuickBooks Database Server Manager relies on to monitor and manage multi-user access to your company file. When it stops running (often after a Windows update, a restart, or the service crashing silently), the host computer can't broadcast the file to other machines — so nobody can open it in multi-user mode, even though the file itself is fine.

How to fix it:

On the host computer, press Win + R, type services.msc, hit Enter.
Scroll down and find QBCFMonitorService in the list.
Check its status — it's likely Stopped.
Right-click it → Start. If it won't start, right-click → Properties → set Startup type to Automatic, then try Start again.
Once it's running, have everyone try reopening the company file.

If it refuses to start or stops again right away:

Open QuickBooks Tool Hub on the host → Network Issues tab → run QuickBooks Database Server Manager again. This often re-registers the service properly.
A quick host computer restart also frequently resolves this if the service is just stuck.

Common triggers: a recent Windows update, antivirus quarantining/blocking the service, or the host machine not fully restarting after a QuickBooks update. Nothing here points to user error on your end.

https://asquarecloudhosting.com/qbcfmonitorservice-not-running-on-this-computer-error/