I had the same issue after upgrading my Windows to Windows Pro 10 21H2. I have two SQL instances - 2014 and 2019 and I was not able to connect only to 2019 one.
I restarted the computer, restarted all SQL related services. Two of the services were showing errors (SQL Server and SQL Server agent) and I was not able to start them.
I could not also start these two services from C:\Windows\SysWOW64\SQLServerManager15.msc
In the logs (C:\Program Files\Microsoft SQL Server\MSSQL15.MSSQLSERVER01\MSSQL\Log) I saw these lines:
Initializing the FallBack certificate failed with error code: 15, state: 29, error number: 0.
Error: 17190, Severity: 16, State: 1.
Unable to initialize SSL encryption because a valid certificate could not be found, and it is not possible to create a self-signed certificate.
Error: 17182, Severity: 16, State: 1.
TDSSNIClient initialization failed with error 0x80092004, status code 0x80. Reason: Unable to initialize SSL support. Cannot find object or property.
Error: 17182, Severity: 16, State: 1.
TDSSNIClient initialization failed with error 0x80092004, status code 0x1. Reason: Initialization failed with an infrastructure error. Check for previous errors. Cannot find object or property.
Error: 17826, Severity: 18, State: 3.
Could not start the network library because of an internal error in the network library. To determine the cause, review the errors immediately preceding this one in the error log.
Error: 17120, Severity: 16, State: 1.
SQL Server could not spawn FRunCommunicationsManager thread. Check the SQL Server error log and the operating system error log for information about possible related problems.
It turned out that after the upgrade the "Log On" property (second tab on service properties) of the service was set to NT AUTHORITY\NETWORK SERVICES. When I changed it to "Local System Account" I was able to start the service and connect to the MS SQL 2019 instance.
This msdn forum post has helped me to find this.