Open the message app in your phone.
Select the conversation with a person from which you can't receive messages.
Tap on menu button.
Tap people & options.
Now make sure that block option is not activated.
If still you have same error than turn off your phone for 10 sec and turn it back on.
After all these attempts if your problem is solved then its OK. If not, then check Lg Nexus 4 Manual for more options.