Facing weird problem

hi mates,

recently, I am facing some weird problems with my forum. such as, PMs are not delivered properly. sometimes users found the whole forum with some codes, which is what appears to me that its script attack by hacker.

But still im confused actually where the problem is. I have installed a new theme and switched on with phbb3 couple of months before does the problem relate to this ?

what can i do to fix this inconvenience and secure the problem.?