I already had this problem before.
Choose "Run" then "Edit Configurations".
In the "General" tab, check the "Deployment Target Options" section.
In my case, the target was already set to "USB Device" and the checkbox "Use same device for future launches" was checked.
I had to change the target to "Show Device Chooser Dialog" and I unchecked the check box. Then my device appeared in the list.
If your device still doesn't appear, then you have to enable USB-Debugging in the smartphone settings again.
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