Hi there-welcome to the forum!
What do you mean they have disappeared? Have you done anything, such as syncing to itunes lately? Maybe you accidentally "unchecked" those apps and they were taken off the ipad and back in the itunes library.
Have you created folders to separate your apps into categories? Or, have you created restrictions to some of the apps? (Settings/General/Restrictions)
Try to restart your ipad by pressing the power switch until you get the lock slide to turn off the ipad. Than turn back on and retry to see if the apps appear.
You may have to reset your ipad-by pressing the power switch and the home button at the same time until the screen turns off. This may take ~5 seconds or so to turn off. Powering back on will give you the apple logo-will take a few seconds to turn on.
See if any of that is your issue. Let us know if it isn't. Good luck.