Magento’s learning curve involves a lot of tears. During development always backup code and database prior to making changes. It’s better to have a stack of wasted, unused burned DVDs than a major failure on a tight schedule with a week old backup. OUCH!!!
First things, go into Magento’s config and turn on ALL logging. Look in var/log for exception.log and system.log, also look in var/report for the latest numbered stack traces, once in a while, you’ll get lucky and it will throw them without visual evidence.
Next step is to put it into Dev mode (index.php edits) and see if the advanced error reporting shows anything. The most dangerous time during development is when you’ve made quite a few changes that have shown on the front end and forgot about a layout change that suddenly gets exposed when you clear the cache. Always run in dev mode with NO CACHING WHATSOEVER so bad code causes immediate grief.
First step, DON’T PANIC as more damage gets done prodding it with a sharp stick in the hopes something will happen than does by stepping back for a couple hours and then starting diagnostic traces to find out what really happened (log messages, dev mode, etc.) I realize it’s too late for that advice, but for others in future…
Full day tomorrow at work kicking a phone answering system running on OS/2 hoping I don’t have the same results so over and out.