What is the error that you are receiving?

It can be so many thinks that is hard to know where to start.

You can try:

Tools - Device Manager.
In the Actions column, click in three dots on the right and then Wipe Data.

or

Search for *.lock folders under .android folder and delete those. This should tell Android studio that the AVD is not running.

or

Remove all *.lock files in the avd folder

Go to File->Invalidates cache then click "Just restart"

Open Android Studio again and launching the emulator should work as it is not lock anymore after deleting the .lock files.