Im a new moderator, my membership is changed few days back, now I can nor approve messages or invite any one?

Hmm, are you also the group owner or is someone else the group owner? If someone else, then it is POSSIBLE that they changed their mind and removed your moderation privileges. In this case, you might email them privately to ask. If you are the group owner, then is there any chance that you have changed your own status? To do that, though, you would have to have taken positive steps to make that happen, namely, you would have to click on Members from the left menu, then under your member name click Edit membership. When the Edit membership screen open, scroll down to Membership Privileges, where you would click Change to Member. Those are the only two options that I can find.