Why is my device not connecting as MTP device using USB cable?

[AC Question]

why is my device not connecting as MTP device using USB cable?

Yesterday i 'd installed CM 12 on my GT_I8190N .....Everything was ok but..... ..when i connected usb to my pc .......the phone was only charging .......not connected as usb storage device <MTP> .......when i looked the setting of USB computer connection then MTP was checked ..... i tried to recheck ..then i said "Unfortunately , setting has been stoped .....
Please help ...urgent ...