Do you see the problem on a MC Client PC, or on the Server?
I have seen it a few times recently after playing live TV on a Client, and doing lots of mucking around. Typically the Server has gone to sleep and then been woken by my TV request. But also sometimes it just happens. I haven't tried to document the events that lead it to happening.
The quick and simple fix is to go into Options/Television/General/Manage Devices, and then exit out again. I can't remember if the fix works when you Cancel out of that dialogue, or if you have to OK your way out of it. I think just Cancel works.
Anyway, if you can document what you were doing exactly, like multiple Clients watching recorded and/or Live TV, what the Server was doing, and so on, Yaobing may be able to find and fix the cause. The issue has been improved over the last few months, but there may still be some problem.
Also share how you have set up MC (Server is HTPC in lounge, Clients in bedrooms, etc.), what OS you are using (Windows what), what version of MC. The current public release is MC21.0.37. You have already listed the tuners, but maybe model numbers would help Yaobing. Even your network setup and components may make a difference. One user was having trouble with a HDHomeRun and it turned out to be his router was the problem. The connection to the tuner wasn't being maintained properly.
Ah. Muzicman0 makes a good point. Even if only the WMC Service that runs all the time in the background is still running, it will cause problems. Even when not being used to watch or record a program, WMC uses a tuner to get OTA EPG data, and seems to "touch" each tuner every now and then, probably to maintain connectivity.