Hot water problem

Hi all ,
I am facing an ongoing issue of hot water system in my house.

I had an shower screen replacement on last weekend. During repairing work we closed main water supply valve of the house to avoid any flooding.
After completing repairing work of my shower we opened our main water supply valve back to normal.

Everything supposed to be normal but i came to know that the water pressure of hot water is completely slow while cold water is fine and normal in every single tap of my house.
e.g. if turn tap to hot water it comes very very slowly and if turn it to cold water the cold supply is completely fine.

My hot water system type is Unvented Cylinder with expansion tank

Can some one suggest me what to do with it ?

Immersion heater with electric thermostat in it?