Try opening the game in windows mode. My boyfriends computer does the same thing, for some reason it wont let him play in full screen. But windows mode its just fine.
On the updater there used to be an option button to change between window mode and full screen, i'm either blind or they took it off.
one way to change it is to go to your C: --> AeriaGames --> Shaiya and open up the config. (the one that has the shaiya icon next to it) you should be able to check windows mode there.
another way is to go to your C: --> AeriaGames --> Shaiya and open up the notepad config. (look at Screenshot below. dont forget to save)
but dont touch anything else if you dont know what your doing!
Also, make sure you are playing in the right screen resolution. To find out what your res is, you should be able to just right click your desktop and choose screen resolution. or go to control panel--> Appearance and personalization--> adjust screen resolution.
Hope this helps.