Gas giants and Ice giants have no mountains.
But when Terra planets or Desert also are flat, then that is not normal.
If you have an NVIDIA graphics card, and have recently updated the drivers, then that could be the reason. There are problems with the latest drivers.
Then it's best to go back to an older driver.
Buf, I have Nvidia (GTX 950) with the lastest drivers lol with win 10 64 bit. How can I go back to another drivers? Can I just install a previus driver?