You have probably already restarted SSDT, but that didn't help. Open the Task Manager (Ctrl + Shift + Esc) and locate DtsDebugHost.exe under Processes and end that process (could be multiple times, end them all). Now try running the package again.
This happens when SSDT/Visual Studio crashes during runtime. If you kill SSDT then the SSIS Debug Host will still be active locking the ISPAC file.