It's a mystery. For a long time I was unable to use my "enter" key to create paragraphs on my laptop, although it worked on my home computer just fine. Then one day it worked again. No idea why.
Go to Settings for your account, click General Settings, then under Miscellaneous Options click Standard Editor. If Standard Editor doesn't help, try the other two Miscellaneous Options -- Enhanced Interface, and Basic Editor. One of those three options should alleviate the problem.