This is most likely to happen if you switch out to another application, and your operating system decides to allocate video memory to that application. Tiny Glade checks the memory budget reported by the graphics driver, and if necessary, squeezes itself to fit. It does this by reducing "resolution scale".
Once video memory (VRAM) is available again, Tiny Glade automatically increases its resolution scale.
You can find the current resolution scale value in the game's settings: hit the Escape key, go to Settings -> Video, and hover over "Resolution scale". On the right panel, you'll see how much VRAM is available to the game, as claimed by your graphics driver.
If you're low on video memory, try closing other applications, or reduce your screen's resolution in the operating system. Specifically if your graphics card only has 2GB of VRAM, and your Windows or Linux desktop runs at 4K screen resolution, there will be very little memory left for 3D applications.
We've also seen cases where graphics drivers lock up some video memory due to their bugs (or operating system bugs), and the system doesn't work correctly until rebooting.
Please also note that if your system has insufficient video memory, Tiny Glade might simply not be able to allocate its buffers and textures, and will either run with greatly decreased resolution, or fail to start altogether; see Tiny Glade's system requirements for the amount of video memory required.