I have a problem with our production database, I think 2 or 3 tables are corrupted. It just came out of nowhere, no major Magento upgrades or big changes made this week, just a few extensions installed. It is currently not possible to backup the database or create a copy of it (it was possible until one week ago).
My questions are two:
1) How could have this happened?
2) How to proceed? I though about running the Magento database repair, but not having a working backup to make a test first (if not an old one) scares me

The tables which seem to be corrupted are sales_flat_order_item and sales_flat_order_payment. They're not big at all, less than 10MB in total, around 9k rows.