"No handler found" message on established install

Hi, I have searched on this topic, but all threads seem to relate to installation problems. I have been running OpenERP 6.1 for 9 months without problem, then today I recieved firstly an error when trying to open invoices RuntimeError: maximum recursion depth exceeded while calling a Python object. I performed a soft reboot of the system, and since then I cannot access anything, I simply get the message No handler found. Any help would be appreciated, as this is a working system, and fundamental to my business!!