Usually when they are just laying down and pop back up when disturbed and then act normally, this is just them being content and relaxing, but there are a few types of flops that can indicate a health problem.
wabbitwiki.com
You can learn to speak and understand Rabbit language.
language.rabbitspeak.com
Flaccid and lethargic flopping where they have very little or no control of their body and can't usually get up or even move. If they are still mentally alert, this can be due to floppy bunny syndrome. Not alert or not conscious, this is other serious health issues, often fatal ones.
wabbitwiki.com
Then there is flopping where they are belly pressing due to their stomach hurting, usually with their butt somewhat pushed up, and their front end may also be somewhat up. They often will also seem restless and keep changing position. They may also be tooth grinding.
wabbitwiki.com