Yeah, this is a common one — and "nothing changed that I know of" is actually really common with this issue too, since the trigger is often something silent (a background Windows update, a firewall rule reset, or antivirus definition update — none of which prompt you or show up as an obvious change).

Since only the host can open it and everyone else times out, the connection between the host and the other machines is being blocked somewhere. Walk through this in order:

Check Windows Firewall on the host computer. This is the #1 cause of exactly this symptom. QuickBooks needs specific ports open (8019, plus a dynamic range depending on your version). A silent Windows Firewall update or a security software update can quietly re-block these overnight without any visible change.
Confirm QuickBooks Database Server Manager is running on the host. Open it and re-scan the folder containing your company file. This re-registers the file and refreshes the network permissions.
Check QBCFMonitorService. Press Win + R, type services.msc, find QBCFMonitorService, and confirm it's set to Automatic and currently Running. If it's stopped, start it manually.
Ping the host from one of the other computers (open Command Prompt, type ping [host computer name]). If this fails or times out, it's a network-level block, not a QuickBooks issue specifically — worth looping in whoever manages your network/router if so.
Open QuickBooks Tool Hub on the host → Network Issues tab → run the Database Server Manager network diagnostic. It's built specifically to catch this kind of "host fine, everyone else blocked" scenario.

https://asquarecloudhosting.com/quickbooks-multi-user-mode-not-working/