I had a problem like this. It wasn't completely locked, but froze up at low temperatures. Turned out it was clogged with gunk from decomposing samples, and all it needed was a good clean. If you're confident about maintaining the instrument yourself, you can try this - start by removing those 6 screws and the plate, then dismantle the furnace cover. Otherwise - or if the problem is different - it's get an engineer in.