Note: For maximum performance make sure the antenna is fully extended and in a vertical, upright position.
You may have ventured into a "Fringe area", or a non-coverage area. (Such as is common for cellular phone users). Check "Signal Strength" from the Quick Status selection in the Minstrel Modem application. If "Low" (low = is less than -100dbm, -50 to -90dbm is good), moving into a different strong coverage area should correct the problem.
Verify that your IP address, also known as the NEI (as assigned by the wireless carrier), is properly entered into the Minstrel. In the Modem Manager Menu, go to "Advanced|Config" and tap on NEI. The 12 digit number that appears should correspond with the number assigned to you by your wireless carrier, if not, simply enter the correct number and tap save. The number should take the format XXX.XXX.XXX.XXX
Verify your side preference (with your ISP) in the "Set A / B Site Preference" setting in the Minstrel Modem Manager Menu under Advanced|Config.
The modem account may be disabled by the wireless carrier. Contact your wireless carrier to have them "Reset Authentication Parameters".