As mentioned above, unscrew the hot and cold PEX line fittings to the faucet under the sink and see if there's flow. Also check that the flow rate is about the same. If good flow in hot & cold, would point to the faucet being the issue.
If you remove the aerator, do you get good flow? If yes, then clean the aerator. If no, the mixing valve could be suspect. Could be plugged up or defective - clean/replace as necessary. If a drop down faucet, can disconnect the flex hose under the sink to see if you get any flow when faucet turned on (use a bucket or pot to collect water). If yes, a plugged or defective drop-down hose - try cleaning or replace as needed.