What's happened to my shower?

2 years ago I had my bathroom done with my bath having mixer taps with a shower hose coming off it. It was working perfectly fine until one day it just stopped. Now, my shower is freezing. With just the hot tap on, the hot water dribbles out. If you turn the cold water on ever so slightly, it completely over powers any hot water. The taps work fine when using the bath. My water tank which I believe is cold water is above in the loft. In my hallway next to bathroom i have a hercal coil indirect cylinder which is hot (not sure if this has anything to do with the shower or hot water, might be heating??) and my boiler is downstairs in the kitchen a stelrad group?? Ideal elan 2?? I don't understand why it worked and suddenly stopped but I did have a leak of cold water from my loft in to my bathroom a few weeks prior to this. Plumber advised "one of the bolts or nuts on the pipe which comes out the tank came loose". This was fixed. My neighbour said she just adjusted the pressure bolts along the pipes under the bath. I'm far too scared to fiddle and don't understand how that would work. As you can tell I don't have a clue what I am on about. Any help would be appreciated. I recently had a plumber round who advised he could pull up my flooring etc to re connect pipes and something else about an electric shower. I don't want this as it worked before. I just want it to work again!