Cannot receive messages but send is OK.

Thunderbird working fine then suddenly went to check email and a "Add security exception" box appeared (see image). Tried "Confirm Security Exception" but no response - although this worked if I unchecked "Permenantly Store this exception" - but following this I could not connect to server (timed out). I thought the problem may be my AVG anti virus so I turned off the email protection. Started Thunderbird again and this time no "Add Security Exception" box but still will not connect to server. One of the advice sections said to check passwords in Tools/Settings/Security but all passwords had disappeared. Sending mail no problem and all account settings correct.