Why has temp on my hot water dropped so much?

We have a Vaillant Turbomax Pro combi boiler. I believe it's only about 4 years old.
My hot water temp has dropped dramatically! I like very hot water, so my temp gage on my combi boiler is at almost max, then hot water taps are very hot normally, but suddenly the past few days the water seems to have dropped to just over luke warm. We noticed the pressure gage had dropped to nothing, and today found the filling loop to top it up but it doesn't seem to have increased the temp of the water at all. The heating is ok, all rediators toasty hot, (except one that we don't ever use, but having tried it we can't get that hot either)
Can anyone advise what's wrond with the hot water? Hopefully something that won't cost us a fortune!
Thanks