I've had the same version of Mathematica 10.4 installed for months without any problem, but this morning it is freezing every time I try to type in a notebook. New notebooks, new kernels, and existing documents all freeze after a few letters.
I have tried restarting several times, deleting configuration files, and updating to no avail. The notebook eventually starts responding after a minute or so, and allows me to type two or three more letters. Curiously, I am able to query Wolfram Alpha (and after I do so, I am able to use the Mathematica input for a few seconds), but still can't type in a new kernel. Might it be a problem with the autocomplete?
Mathematica starts using more and more memory after it freezes, increasing to about 70 or 80% before it starts responding again.
Is this a common problem? Any ideas? Happy to provide more information.
I am running Windows 10 (x64 on an AMD CPU) with 8GB of RAM.
Edit: Deleting configuration files in C:\Users...\AppData\Roaming\Mathematica worked.