Remove the outlet valve, maybe something made its way in and is blocking the water flow. You could also carefully remove the flushpipe from the back of the WC and cistern to make sure there is no blockage there and/or the washer hasn't moved in any way and is blocking the water flow. Give it a good clean and while your at it you could replace the washer on the valve. If you haven't modified anything on the WC and the waterflow was OK before then I'm sure you should be able to get that "oomph" back.

Hello,

please suggest me, actually I have an office in building at 2nd Flore and when my office was remolding, that time i have the plumber attached a water tank, and add all the pipe line with that, and he added an hand shower flush with WC, that the water pressure was good going, but now it's not working properly and pressure is very low.