Welcome to health care magic forum.
Cause for occurrence of ulcers on roof of mouth has to be evaluated by a dentist
If there is trauma,periodontitis
or deep caries associated and left untreated,the infection can spread into tissue spaces causing sores as well as into sinus spaces.
I would suggest you to get the sore drained and curetted.
Meanwhile,maintain oral hygiene
Take nutritious diet,avoid smoking
,spicy foods etc.
Hope this helps.