Plumber was here then expansion tank leaking......ugh

Huck2Spit

We're having an issue with our showers where the water wouldn't stay hot throughout the whole shower. I called a plumber who's worked on her boiler before we have a combi system. He suggested replacing the blending valve which he then did and said let us know if the problem persists. I went to take a shower a couple hours after he left and there was no hot water! Then I went and checked other taps around the house and there was no hot water anywhere. I then went and checked the boiler itself and noticed a leak coming from a top the expansion tank at the release valve there ( I think ). 
So I called the plumbing service but being after 4:00 got there after hours answering service.. a tech call me back it was not helpful at all and basically gave me the rates for coming out after hours and suggested I might need a new boiler. 
 He did not seem to understand that this boiler which hasn't leaked for years all of a sudden was leaking a couple hours after one of their techs worked on the system...   Anyway it was a drip of a leak and I put a bucket under it. 
They told me that they call me in the morning to figure it out to avoid the after-hour charges, ugh!  
Later on last night I noticed our hot water baseboard heat wasn't coming on so I went to check out the boiler again this time noticing it wasn't on at all the leak had caught my attention earlier but now nothing was happening it seemed to be powered off. 
I looked around a little further and saw a fuse sitting on a table adjacent to the boiler...
So I reinstalled the fuse into the red junction box above the boiler and ,what do you know, the thing fired right up the heat came on and the mixing valve replacement seem to fix the hot water issues.  
So I guess my question is how does what this plumber /tech did or didn't do cause my expansion tank to now leak?  The tank itself was replaced in the past 2 years and the leak/drip seems to be coming from the release valve. 
Would water drip from there if the system was shut down not restarting properly? 
 I don't know but these guys made one neglectful error I'm not replacing a simple fuse and because it's a little older unit they several times mentioned the problem could be I need to replace the whole unit itself so I'm a little skeptical.

Jamie Hall

Exactly where is the water coming from? Is it a genuine leak? Is it coming from the Schrader valve on the tank? Is it coming from a pressure relief valve? And is this on the domestic hot water side of the combi, or the heating side?

EBEBRATT-Ed

You need to post some pictures we don't have x ray vision. We need to see what is going on and how it is piped.

hot_rod

A boiler expansion tank, or a DHW thermal expansion tank?