Hi! i usually masturbate, but today my penus just wont erect what so ever! what causes this problem?

No erection: There may be many reasons for this but if it has only occurred once i don't know that it is a problem. If you are seeing repitition of this or are concerned i would contact your physician.

No particular issue: To worry about unless it continues. The inability to obtain an erection and complete masturbation on a particular day is not at all unusual and is a function of your libido, attention, preoccupation, stress level, and degree of somnolence on a particular day. Don't worry if things have been functioning well 90+% of the time. You are perfectly normal.