Cannot is the standard full form in modern English. In most sentences, if you mean be unable to, write cannot as one word: I cannot attend today.
Can’t is simply the contraction of cannot. It means the same thing, but it is more informal: I can’t attend today.
Can not as two words is much less common. Use it when not belongs to a separate construction rather than directly to can. The most common example is not only: You can not only attend, but also present.
A useful quick check is this: if you can replace the phrase with cannot and keep the same meaning, use the one word form. If not is part of something like not only, two words may be correct.
- Cannot: We cannot park here.
- Can’t: We can’t park here.
- Can not: She can not only sing, but also compose.