Garden tap not working

Could be 1 of 2 things the washer in the outside tap is stuck down or the valve under sink is not opening fully

The majority of garden/outside taps are now supplied with an integral check valve which is required for current water regulations. It is very common for this check valve to collapse inside which in turn restricts the flow. Purely replacing just the tap itself will cure the fault. If your fairly competent at DIY this may be something you can do yourself, if not it would be a straight forward replacement for a plumber.

Seal stuck in the outside tap, to test turn the water feed off under the sink , then unwind the outside tap from the back plate fixed to the wall , turn the water back on under the sink and the water should blast out , this will establish if the tap has a faulty seal and needs replacement. Thanks Matt

This is probably due to the spindle on the outside tap not opening fully as they do clog up or rust over time, either that or there is a blockage in the tap, I would recommend a new tap as they are not that expensive

the gland is stuck so the flow of water isn't happening. Try turning off the water and removing the tap head section to see if the washer is blocking the flow.

Valve or washer needs replacement, possibly the tap it’s self depending on condition

First think to do take off the outside tap and see if it's the check valve in the tap itself which has blocked up. If it's the tap when you unscrew it water should pour out. You will need some plumbers tape to wrap around it before putting back on. If it has easiest thing to do replace outside tap or disassemble and unblock. Normally taps here have a check valve in them to prevent back flow of water from hose etc. If it's not the outside tap then trace it back inside. Normally you would not have the check valve inside.

It sounds like the jumper has become stuck inside the tap. Could be a simple fix if you can isolate the water supply to it but more easier to just renew the tap itself for a new one or Lever bib tap.

This sounds like the spindle has seized up where the washer seats. Potentially could be stripped down or the tap part replaced