"Failed to initialize the Common Language Runtime" when trying to access a SQL 2014 Express database

I'm currently developing a website that uses a SQL Server 2014 Express database on my machine. This has always worked in the past, but today I got the following error:

Failed to initialize the Common Language Runtime (CLR) v4.0.30319 with HRESULT 0x80131534. You need to restart SQL Server to use CLR integration features.

Of course I restarted SQL Server 2014 Express, but to no avail.

I ended up downloading a .NET framework repair tool. After running it and rebooting my computer, I was able to connect to the database again.