How do I re-enter my gMail account password in settings?

I have iPad connected to WiFi .I have version 8.3 (12F69). For the past two days I have not been able to send messages form my iPad. the messages are marked as "not delivered" with exclamation mark. when I re-try to send the message. It fails and I get a message stating "cannot access your Gmail account, Please re-enter account Password for ......in settings. I click on setting and click"Re-enter password" that is highlighted in blue. I am in Gmail screen under Accounts.google.com but it runs forever and returns to the previous screen where I have to re-enter the password ... It's just going around in circles.