I have been building an advanced Search visualforce application for weeks now using Force.com IDE in eclipse.  The originally did it on the page but our SF account was upgraded to production and I was no longer able to access the APEX code on the page.  So I have been deploying to server everytime I made a change and wanted to do testing on our SF instance.  Just today I tried to deploy after some changes and got the error message,

" Average test coverage across all Apex Classes and Triggers is 0%, at least 75% test coverage is required".

This has never appeared before and I am stumped as to why it just appeared suddenly and it not allowing me to deploy my code anymore.  Did something change recently?  I have never written any tests for this and I am not sure even how to write tests in Apex.