Solution 1:
- Close Android Studio
- open task manager
- Kill adb process (on Task Manager on Windows)
- Open Android Studio
Solution 2:
- turned OFF your Wi-Fi
- open
C: \Users\USERNAME\AppData\Local\Android\Sdk\platform-tools
open command prompt on given address
type
adb disconnect
command in Command prompt

- Restart Android studio
Solution 3:
Go to SDK folder delete platform-tools folder.. invalidate and restart the studio
Solution 4:
Some times you have Anti-Virus in your P.C (Personal Computer) that's why loading Device issue comes into play. In this case,Turn off Anti-Virus and restart your P.C (Personal Computer) and run Android Studio your issue will be solved.
Note: if these solution are not working then let me know i'll fix for you...