You have a 30 day trial period starting with the installation. After that, some functions - i.e. the Develop module - are deactivated.
This can not be reactivated.
If you see a blue screen with a white cross, then it's a GPU issue and not the end of the trial. Please deactivate the GPU acceleration in this case: Adobe Photoshop Lightroom Help | Lightroom Graphics Processor Acceleration (GPU) Troubleshooting & FAQ