No Response from Server

Got this response while joining user to the MUC Room:

No response from server.:

at org.jivesoftware.smackx.muc.MultiUserChat.join(MultiUserChat.java:498)

at org.jivesoftware.smackx.muc.MultiUserChat.join(MultiUserChat.java:392)

Above error is followed by:

java.lang.IllegalStateException: Not connected to server.

at org.jivesoftware.smack.XMPPConnection.sendPacket(XMPPConnection.java:445)

It works for sometime but later these above error comes.