More info plz. Is it stuck in the drain ? Ie. can you see it ? Or has it gone down the drain and is stuck in the pipe ?

Under the sink you should have a curve that goes down from the drain, then comes back up, then into the wall. You should be able to unscrew two places and take the “macaroni” shaped part out, and clean out the whole vertical part.

First shut the water off. This is just in case you bump the faucet levers, not strictly necessary, but a good safety net for a mess.

Second, put a shallow bowl underneath, as some water will come out.

Third, unscrews the two nuts that hold the lower curved piece in place. These parts will stay on the wall side (just slide them up and out of the way), and will come off the vertical piece. As you do this; note where the sealing rings/washers are and which direction they face.

Fourth, if your sink has a drain plug you control from above, you may need to unscrew the small control arm under the since that connects to the drain plug assembly. It’s a steel rod with a small plastic ball on it. Remove that part first, and then lift the plug out from the sink.

Fifth, at this point you should have a clear, open tube between the sink and your bowl underneath. Taking whatever you have to push and scrub to clean it out.

Sixth, put it back together in reverse order. Some Teflon tape on the drain control “arm” where it screws into the faucet would be wose

Another win for the shop-vac.

If it’s stuck down in the U bend undo it as already described:
If it’s a big cap and it’s sitting just below the plug hole you’ll need to either: poke it up from below (after undoing the u bend) or poke a hole in it from above and hook it out.
If it’s wet it’ll be very hard to make anything stick to it but if you can get a wire coat hanger and undo the twisted ends you can heat the end with a lighter and poke it straight down through the plastic cap. Then twist it so the wiggly end goes in like a corkscrew and pull the cap out.

Long tweezers are good for this kind of thing if you have any.

An easier solution to try may be to use a flexible claw grabber pickup tool if you have one or have an easy way to get one. Will save the mess of cleaning out the p trap and getting everything re-sealed without a leak.

Ok, we need an update. What ended up happening?