I just had this issue, and Googling didn't yield a ton of results when I looked it up. Alchete's answer was what solved this issue for me, but being new to the Windows command line, I didn't know how to execute it. So I just want to elaborate in case anyone is in the same boat as I was.
Open the Start Menu and type cmd to get to the command line.
Change directories to get to the sdk folder that contains the adb.exe file.
For me the command looked like this:
cd DocumentsProgrammingandroid-studiosdkplatform-tools
Adjust based on the path to your sdk folder.
Then, you can enter:
adb kill-server
Followed by:
adb start-server
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