Could be the tap washer has broken down, or came away from the spindle. Or could just be the plastic spray cap could be blocked. The plastic cap is fitted in the tap (where the water comes out) pop it out using a screwdriver or something similar and try the tap see if its any better.
If not, turn off water and take the tap head off, there'll be a hexagonal head where it meets the tap, undo this and replace the washer and clean the tap out.
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