How do I check what has gone wrong with my immersion heater

Hi in some cases this would trip the electric's as some times the element blow's on the immersion heater this would be about a 2 hour job for a plumber to drain tank and replace thankyou Shaun

If you need to ask then in my opinion it will not be safe for you to try. If the fuse has blown then there is almost certainly a reason for this which would normally be a failed element. A faulty thermostat usually fails in the closed circuit position meaning your immersion would not turn of. Advice at least get someone who understands electrics if not a registered electrician.