The brush should be smooth or pointed, but suddenly it becomes like this:'( Any suggestions? Please help me 
The brush preset you have selected does not use pressure for size but for opacity. However opacity also doesn’t seem to work which indicates you have accidentally disabled pressure or something is wrong with your pen or graphics tablet.
Every now and then, my graphics tablet seems to stop responding to pressure. All I need to do is disconnect it, reconnect it, and restart Krita for it to start working again.
Oh, it finally worked. Turns out the problem was with the graphics tablet. Thank you so muchh 