Our cold water supply to the main toilet (upstairs) has stopped. This seems to have occurred after a sink unit in the en-suite was re-plumbed and fitted.
It is a push button type toilet and have checked for debris blocking any inlets.
I have now disconnected everything from the cistern and just have a flexi-pipe connected to the cold water pipe. But still no water.
Could it be a problem with a water tank in the loft ?

Sounds like you have a pressure assist toilet there mwdav... see pic.

If that is the case..these can be very complicated to work on.

First, when you disconnect the water supply from the toilet itself.. is there any water coming out of flexible connector?

Test by shutting toilet shutoff, off, then disconnect the flexible connector and directing flexi. Connector into bucket, then turn on water shutoff under toilet and see if flows well. If it doesn't flow well then problem is probably at the shutoff itself.

If not flowing out of toilet shutoff, shut water upstairs in loft and drain water in house to lowest fixture (open faucets) and then unscrew the shutoffs packing nut (see pic.#2) and remove the stem assembly... then have a friend pulse the water back on for a second to clear of sediment (cover shutoff with rag)... re-install the stem, tighten packing nut, open the valve and turn water on to whole house again (purge air through all faucets). Then try flushing toilet.. see if that fixed it!

Most likely issue not in the loft (most likely) as water in loft feeds all fixtures and would expect more issues then you mentioned.

If water flows from the flexi. well, then issue is up in tank and you need to call a plumber to help with this!!

Try these things... get back to me with thoughts... Mark

If this answer helped please RATE THIS ANSWER. Thank you.

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Thanks for the quick response massplumber. I don't have a pressure assist toilet after looking at your pics.
The make and model of my system is a Laufen Pro.
I have taken away the shut off valve to the flexi-pipe and connected the flexi direct to the incoming water pipe.
No water is coming through that.
I guess the water supply to that pipe is blocked somewhere.

Are you saying that the flexi. Line is clogged? That's not really possible... really?? If it clogged, take coat hanger and push through.. that will dislodge debris.. or just replace.

BUT I MUST HAVE MISUNDERSTOOD!!! I AM SURE.

Did you disconnect the water to toilet and confirm there was water up to the shutoff???

By the way.. did plumber install that sink in the en-suite? If so, call him back and ask him to fix it! That is part of any job -->> restoring water back to all fixtures after job is done!

Let us know... Mark

I've now taken the flexi off the incoming water pipe - so nothing is now connected and no water is flowing through.
I have also turned all taps off in the loft area and turned them on again but still no luck.

OK.. did you try to shut all taps in the loft and then remove the stem from the toilet shutoff like I posted in post #2?

Shut water upstairs in loft and drain water in house to lowest fixture (open faucets) and then unscrew the shutoffs packing nut (see pic.#2 in post #2) and remove the stem assembly... then have a friend pulse the water back on for a second to clear of sediment (cover shutoff with rag)... re-install the stem, tighten packing nut, open the valve and turn water on to whole house again (purge air through all faucets). Then try flushing toilet.. see if that fixed it!

Just curious?