My apple pen is not working

First, compatibility. Verify that your Apple Pencil is compatible with your iPad:

Apple Pencil compatibility - Apple Support

Assuming that you have an Apple Pencil that is compatible with your iPad, this support page may be helpful with pairing and use of the Pencil with your iPad:

Connect Apple Pencil with your iPad - Apple Support

If you continue to experience difficulties, here is one of the better troubleshooting guides:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

Be aware that if your Pencil has been set-aside for an extended period, unused and uncharged, the Pencil battery may have been damaged through deep discharge. In common with all Li-ion batteries, they must always have some level of charge.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. 

It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.

If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced by your retailer or at an Apple Store.

Gentle warming of the Pencil (such as placing on a hot water radiator), prior to attempting charging of the Pencil, can sometimes resurrect a dead Pencil battery.

There is no detail here to be able to offer any sort of reasonable assistance. What have you tried, what version Apple Pencil is it? What version iPad is it? There could be any number of issues that may be causing it. However, the first issue that comes to mind is maybe the battery has died on the Apple Pencil.

If the pencil has been sitting around and has not been charged for an extended period of time, it may have caused the battery to deplete too much and will no longer take a charge. There are many different pages that come up with a Google search that you can try to fix common issues. If there is any additional information that can be detailed it may help with getting successful assistance.

Also check out Writing an effective Apple Support Communities question - Apple Community for some information on what to post to get the most effective assistance.