This is probably because demo has expired for this or another application protected with same application signature and program id (since they are sharing same license data) on this computer.
Change application signature and protect application again in order to get a clean start. You can also increase Update ID and enable "Reset demo mode for this update" option in demo mode settings and protect new update. which will reset demo limitations on remote computer.
If you are testing different protection scenarios with PC Guard on your local machine you can use "Reset license" button in general project setting to reset current license data stored on your computer.