We have a neptune bar mixer shower that has been fitted to the wall and plumbed. All the water has been turned back on, the hot water is working fine but nothing is coming through the cold water side. cold water is working though all the other taps. what might be the problem? the shower did not come with a manual either!

You have a blockage on the hot water inlet to your shower valve . If all other hot oulets are fine . Take the valve out and check the mixer body for blockage . Or do you have a valve on the hot side that is not fully on .

It's doubtful your water heater unit is the problem. There is simply no reason for it to only work on sinks unless some how the pipe size was drastically reduced coming out of the unit so sinks got plenty of hot water but the shower does not. Take a second look at the installation. The pipe should not be less than 1/2" coming out. If that's OK then consider a clog or valve problem.

you make no sense. From the picture the water "mixer" is the center lever or is it a diverter valve for the shower and water nozzle below???
The knobs are for hot and cold water??

You need a new cartridge for the shower located behind the handle.

may have a screwed up washer seal on the hot side..it will stop the flow to a degree and allow more cold water in..calls for dismantling the entire control or replacing it..

It is the shower head or plumbing that has a built in mixer or reducer.

The problem is not the water heater. The shower has a mixer or a limiter built into the plumbing so that the water wont get hot. It is a safety feature to prevent burns and protect children from scalding.

Type of water heater is unknown
If you have single-handled faucet the faucet cartridge might be bad.
If you have recirculation system, then check valve might be bad.
Copy following links for troubleshoot.
http://waterheatertimer.org/Crossover.html
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

It is in the mixer system,you may have put in repair kits with new springs,diaphrams,and o-rings or seals(washers).

from a plumber all hot and cold connections on all major showers are hot on the left and cold on the right just like the taps on your sink or washbowl