Recently, I have faced the same problem twice. Each time a pop up window appears like this figure that contains the message.

Don't be disappointed, be relaxed, because it remains effective only for 1 day. You need to wait, and on the next day, you will get full access as previous. The cause of this unexpected message is remaining connected with GPU for a long time without utilizing it (i.e. not running the code that needs GPU). More details, the causes may be one of the followings:

Let's say, you are connected with GPU, then run a code that takes 40 mins to execute. But you leave your laptop and go to sleep for 2, 3 or more hours. Then this unexpected message can appear.

If you don't use GPU but remain connected with GPU, after some time Colab will give you a warning message like Warning: You are connected to a GPU runtime, but not utilising the GPU. Change to a standard runtime. A good practice is to change the runtime on that time, otherwise, you may get blocked on this day.