Losing levels just basically means you have become weaker in all senses of the word.
If you are a mage, your weakness is reflected in the lack of skill or concentration (perhaps memory) to cast those higher level spells.
If you are a priest, it could mean a lack of focus or faith needed to cast those mighty divine magics. I wouldn't say the god is disappointed with him because that would be the result of something else, not level draining.
