Brachos 5a states:
אמר רבא ואיתימא רב חסדא, "אם רואה אדם שיסורין באין עליו יפשפש במעשיו, שנא' ,(איכה ג, מ) 'נחפשה דרכינו ונחקורה ונשובה עד ה'.' פשפש ולא מצא יתלה בבטול תורה שנאמר (תהלים צד, יב) 'אשרי הגבר אשר תיסרנו יה ומתורתך תלמדנו.' ואם תלה ולא מצא בידוע שיסורין של אהבה הם, שנאמר (משלי ג, יב), 'כי את אשר יאהב ה׳ יוכיח.'"
Rava, and some say Rav Chisda, said, "If a person sees that suffering has befallen him, he should examine his actions, as it is stated (Eichah 3:40), 'We will search and examine our ways, and return to G-d.' If he examined his deeds and did not find anything, attribute it to Bitul Torah, as it is stated (Tehillim 94:12), 'Happy is the man whom You punish, Hashem, and teach out of Your Law.' And if he did attribute [it to Bitul Torah] but did not find [that to be so], it must be afflictions of love, as it is stated (Mishlei 3:12), 'For whom Hashem loves, He rebukes.'"
To summarize: When one is suffering, he should (1) see if he has any sins. If he doesn't, (2) it must be Bitul Torah, and if that's not true, either, (3) it must be afflictions of love.
What are "afflictions of love"? Later down on the same amud, the Gemara records a debate between R' Yaakov bar Idi and R' Acha bar Chanina (it's not sure who said which) that either it's afflictions that don't prevent Torah learning (but could prevent davening) or afflictions that don't prevent davening (but could prevent Torah learning). There is a third opinion brought there (R' Abba bar R' Chiya bar Abba in the name of his father in the name of R' Yochanan) that afflictions of love could prevent both.
What if one is suffering, but able to daven and learn, and is unable to attribute it to any sin in particular? What kind of suffering is that? It's not an atonement for sin, nor is it an affliction of love, so what is it?
To give an example of what I am talking about, say a person is in the hospital. He's able to daven, and he's able to learn, but he's not able to do much else.